remove unused type parameter

[palday]

Julia 1.8.2 warns about unused type parameters

closes #479, closes #478

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (b4afc92) 76.86% compared to head (235814f) 76.86%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #456 +/- ## ======================================= Coverage 76.86% 76.86% ======================================= Files 25 25 Lines 1664 1664 ======================================= Hits 1279 1279 Misses 385 385 ``` | [Impacted Files](https://codecov.io/gh/JuliaInterop/RCall.jl/pull/456?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaInterop) | Coverage Δ | | |---|---|---| | [src/convert/base.jl](https://codecov.io/gh/JuliaInterop/RCall.jl/pull/456?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaInterop#diff-c3JjL2NvbnZlcnQvYmFzZS5qbA==) | `90.39% <100.00%> (ø)` | | 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=JuliaInterop). 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=JuliaInterop)

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[palday]

The Windows failures look unrelated and just due to some connection flakiness. @randy3k could you restart them?

[ViralBShah]

There seems to be a problem with the R install on Windows.

[ViralBShah]

Ok to merge?