Some misc. cleanup

[bretambrose]

• Remove the unused local subscribe API and logic
• Rename connection to adapter in places to make clear the distinction between the implementation (adapter) and the polymorphic base (connection)
• Remove support for building without websockets. This hasn't worked in over two years and is not something we want to support going forward.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Patch coverage: 40.86% and project coverage change: +0.78 :tada:

Comparison is base (305239f) 79.56% compared to head (f859340) 80.34%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #292 +/- ## ========================================== + Coverage 79.56% 80.34% +0.78% ========================================== Files 18 18 Lines 7639 7571 -68 ========================================== + Hits 6078 6083 +5 + Misses 1561 1488 -73 ``` | [Impacted Files](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/292?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs) | Coverage Δ | | |---|---|---| | [source/client\_impl\_shared.c](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/292?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL2NsaWVudF9pbXBsX3NoYXJlZC5j) | `80.95% <ø> (+3.67%)` | :arrow_up: | | [source/mqtt.c](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/292?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL21xdHQuYw==) | `94.59% <ø> (ø)` | | | [source/mqtt3\_to\_mqtt5\_adapter.c](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/292?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL21xdHQzX3RvX21xdHQ1X2FkYXB0ZXIuYw==) | `42.50% <33.33%> (ø)` | | | [source/client.c](https://app.codecov.io/gh/awslabs/aws-c-mqtt/pull/292?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#diff-c291cmNlL2NsaWVudC5j) | `69.03% <80.00%> (+3.56%)` | :arrow_up: |

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