[CMAKE] Remove environment variable support from the add external components script

[dbarker]

Fixes #3409

Changes

• Removes OPENTELEMETRY_EXTERNAL_COMPONENT_PATH and OPENTELEMETRY_EXTERNAL_COMPONENT_URL environment variable support from the cmake/opentelemetry-build-external-component.cmake script

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.84%. Comparing base (1accbc0) to head (fc024e8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4211   +/-   ##
=======================================
  Coverage   82.84%   82.84%           
=======================================
  Files         415      415           
  Lines       17423    17423           
=======================================
  Hits        14433    14433           
  Misses       2990     2990           

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