[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 82.79%. Comparing base (c0dc559) to head (cecdc8b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4211      +/-   ##
==========================================
- Coverage   82.80%   82.79%   -0.01%     
==========================================
  Files         415      415              
  Lines       17452    17452              
==========================================
- Hits        14449    14447       -2     
- Misses       3003     3005       +2     

see 1 file with indirect coverage changes

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[dbarker]

@ThomsonTan can you take a review pass?

OPENTELEMETRY_EXTERNAL_COMPONENT_PATH CMake variable is used by the opentelemetry-cpp vckpg port, but the environment variables for OPENTELEMETRY_EXTERNAL_COMPONENT_PATH and OPENTELEMETRY_EXTERNAL_COMPONENT_URL appear to be unused.