[SPARK-55383][INFRA] Only send test report to codecov in coverage run

### What changes were proposed in this pull request?

Instead of trying to send test report on all main branch commits, we only send it for coverage run.

Also removed an unused coverage token.

### Why are the changes needed?

We tried the test dashboard of codecov - https://app.codecov.io/github/apache/spark/tests but it's not super great.

* It can't parse the xml file to locate the failed test case. Instead it records a failure for the full test suite.
* It does not have a timeline so we don't really know if the test failed recently.
* The deduplication is bad.
* It messed up our coverage report - because the backend can't split the test and coverage report into their own category.

However it's not completely useless. The average time of tests actually helps us to locate slowest tests. It does not mess up with coverage report, as long as for each commit we have both the coverage report and test report.

So, we can do a report only on coverage run - single report each day for both coverage and test speed.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

CI should pass, hopefully coverage can recover.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #54168 from gaogaotiantian/revert-test-report.

Authored-by: Tian Gao <gaogaotiantian@hotmail.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>