[BUILD] Do not set OTELCPP_PROTO_PATH in the CMake cache

[marcalff]

Do not set OTELCPP_PROTO_PATH in the CMake cache.

This is an oversight from:

• [Build] Bring your own dependency: opentelemetry-proto #1730

Setting OTELCPP_PROTO_PATH in the cache prevents to use this flag as intended.

Changes

Please provide a brief description of the changes here.

• Do not set OTELCPP_PROTO_PATH in the cmake cache, so it can be set by the calling makefiles.

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

[netlify]

✅ Deploy Preview for opentelemetry-cpp-api-docs canceled.

Name	Link
🔨 Latest commit	fb99a61
🔍 Latest deploy log	https://app.netlify.com/sites/opentelemetry-cpp-api-docs/deploys/67406c03140357000846b3a6

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.86%. Comparing base (fe68d51) to head (fb99a61).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3160   +/-   ##
=======================================
  Coverage   87.86%   87.86%           
=======================================
  Files         195      195           
  Lines        6151     6151           
=======================================
  Hits         5404     5404           
  Misses        747      747           

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