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 79.61%. Comparing base (3eedf1e) to head (7e66f12).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #518   +/-   ##
=======================================
  Coverage   79.61%   79.61%           
=======================================
  Files          27       27           
  Lines       11702    11702           
=======================================
  Hits         9316     9316           
  Misses       2386     2386           

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