chore(ci): remove codecov (#691)

Remove Codecov configuration and CI steps for uploading coverage/test
reports.

## Motivation and Context
- Codecov integration relies on third-party credentials
- Codecov has not demonstrated clear value for the project:
- Coverage analysis is already handled by kover, which runs locally and
in CI
- PR reviews still require manual inspection of tests regardless of
codecov reports
- Codecov flaky test detection is unreliable, it marks 3325 out of all
tests as flaky with similar failure rates, making the signal useless
(https://app.codecov.io/gh/modelcontextprotocol/kotlin-sdk/tests)

context7.json

@@ -12,7 +12,6 @@
     "CONTRIBUTING.md",
     "LICENSE",
     "SECURITY.md",
-    "codecov.yml",
     "gradlew",
     "gradlew.bat",
     ".editorconfig",