Remove clang-3 from CI

[sbSteveK]

Remove clang-3 from CI

CI uses aws-crt-builder, which when instructed to use clang-3, has failed to find it and has been falling back on GNU 7.5 for the past year+. This means while clang-3 is listed in our CI as having been tested, it isn't. Fixing aws-crt-builder to use clang-3 has revealed a large number of build failures across aws-c-* libraries. Lack of issues related to this implies that clang-3 is not in use and can be removed from CI.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 89.72%. Comparing base (fbd8a91) to head (3060798).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #520   +/-   ##
=======================================
  Coverage   89.72%   89.72%           
=======================================
  Files          20       20           
  Lines        6382     6382           
=======================================
  Hits         5726     5726           
  Misses        656      656           

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