attribute: Drop backport macros and port to native C++17 attributes

[stotko]

Since raising the minimum required standard to C++17, the attribute macros will always point to the native attributes. Drop the attribute header and port all uses of the macros to the native C++17 attributes. Furthermore, drop the STDGPU_HAS_CXX_17 macro which is only used in the attribute header and also no longer needed.

Partially addresses #314

[codecov[bot]]

Codecov Report

Base: 97.15% // Head: 97.15% // No change to project coverage :thumbsup:

Coverage data is based on head (306460a) compared to base (36e7ea6). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #323 +/- ## ======================================= Coverage 97.15% 97.15% ======================================= Files 32 32 Lines 2352 2352 ======================================= Hits 2285 2285 Misses 67 67 ``` | [Impacted Files](https://codecov.io/gh/stotko/stdgpu/pull/323?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko) | Coverage Δ | | |---|---|---| | [src/stdgpu/bitset.cuh](https://codecov.io/gh/stotko/stdgpu/pull/323/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS9iaXRzZXQuY3Vo) | `100.00% <ø> (ø)` | | | [src/stdgpu/deque.cuh](https://codecov.io/gh/stotko/stdgpu/pull/323/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS9kZXF1ZS5jdWg=) | `100.00% <ø> (ø)` | | | [src/stdgpu/impl/unordered\_base.cuh](https://codecov.io/gh/stotko/stdgpu/pull/323/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS9pbXBsL3Vub3JkZXJlZF9iYXNlLmN1aA==) | `100.00% <ø> (ø)` | | | [src/stdgpu/impl/vector\_detail.cuh](https://codecov.io/gh/stotko/stdgpu/pull/323/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS9pbXBsL3ZlY3Rvcl9kZXRhaWwuY3Vo) | `94.00% <ø> (ø)` | | | [src/stdgpu/mutex.cuh](https://codecov.io/gh/stotko/stdgpu/pull/323/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS9tdXRleC5jdWg=) | `100.00% <ø> (ø)` | | | [src/stdgpu/ranges.h](https://codecov.io/gh/stotko/stdgpu/pull/323/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS9yYW5nZXMuaA==) | `100.00% <ø> (ø)` | | | [src/stdgpu/unordered\_map.cuh](https://codecov.io/gh/stotko/stdgpu/pull/323/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS91bm9yZGVyZWRfbWFwLmN1aA==) | `100.00% <ø> (ø)` | | | [src/stdgpu/unordered\_set.cuh](https://codecov.io/gh/stotko/stdgpu/pull/323/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS91bm9yZGVyZWRfc2V0LmN1aA==) | `100.00% <ø> (ø)` | | | [src/stdgpu/vector.cuh](https://codecov.io/gh/stotko/stdgpu/pull/323/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS92ZWN0b3IuY3Vo) | `100.00% <ø> (ø)` | | | [src/stdgpu/impl/atomic\_detail.cuh](https://codecov.io/gh/stotko/stdgpu/pull/323/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko#diff-c3JjL3N0ZGdwdS9pbXBsL2F0b21pY19kZXRhaWwuY3Vo) | `99.59% <100.00%> (ø)` | | | ... and [2 more](https://codecov.io/gh/stotko/stdgpu/pull/323/diff?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko) | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.