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)

.github/workflows/build.yml

@@ -104,22 +104,6 @@ jobs:
           report_paths: '**/test-results/**/TEST-*.xml'
           truncate_stack_traces: false
 
-      - name: Upload coverage reports to Codecov
-        if: ${{ !cancelled() && matrix.os == 'ubuntu-latest' }}
-        uses: codecov/codecov-action@v6
-        with:
-          token: ${{ secrets.CODECOV_TOKEN }}
-          report_type: coverage
-          directory: ./build/reports/kover
-          files: report.xml
-
-      - name: Upload test results to Codecov
-        if: ${{ !cancelled() && matrix.os == 'ubuntu-latest' }}
-        uses: codecov/codecov-action@v6
-        with:
-          token: ${{ secrets.CODECOV_TOKEN }}
-          report_type: test_results
-
 
       - name: Archive Maven Local Repository
         if: ${{ success() && matrix.os == 'ubuntu-latest' }}