[chore][tests/receiver/kong] Remove redundant tests

[crobert-1]

Description:

This was a golden test for metrics coming from a Kong gateway, by way of the Prometheus receiver. None of this functionality is specific to our distro, so the test was moved upstream in open-telemetry/opentelemetry-collector-contrib#42741. Now that the test exists upstream we can remove from our repo.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.71%. Comparing base (f854fd4) to head (343deb3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6775      +/-   ##
==========================================
- Coverage   43.72%   43.71%   -0.02%     
==========================================
  Files         376      376              
  Lines       26363    26363              
==========================================
- Hits        11527    11524       -3     
- Misses      14013    14015       +2     
- Partials      823      824       +1     

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[pjanotti]

LGTM, the failure at first sounds unrelated, but it would be good to confirm before merging:

2025-09-17T15:24:39.2191080Z === RUN   TestConfigServer_RequireEnvVar
2025-09-17T15:24:39.2191359Z     config_server_test.go:55: 
2025-09-17T15:24:39.2192002Z         	Error Trace:	D:/a/splunk-otel-collector/splunk-otel-collector/internal/configconverter/config_server_test.go:55
2025-09-17T15:24:39.2192662Z         	Error:      	An error is expected but got nil.
2025-09-17T15:24:39.2193089Z         	Test:       	TestConfigServer_RequireEnvVar