Changes

fix(pingcap/tidb): update unit test pipeline to remove coverage upload step for enterprise extension (#4029)

This pull request removes the post-success step that uploaded code
coverage data to Codecov after running unit tests. This means coverage
data will no longer be automatically uploaded for this pipeline.

Testing and coverage reporting:

* Removed the `post { success { ... } }` block that uploaded coverage
data to Codecov after successful unit tests in `ghpr_unit_test.groovy`.


It will cause parsing failure on codecov to upload coverage data for
private folder on public repo.
 (commit: 4c7dc0d)