[SPARK-7721][INFRA][FOLLOW-UP] Remove cloned coverage repo after posting HTMLs

## What changes were proposed in this pull request?

This PR proposes to remove cloned `pyspark-coverage-site` repo.

it doesn't looks a problem in PR builder but somehow it's problematic in `spark-master-test-sbt-hadoop-2.7`.

## How was this patch tested?

Jenkins.

Closes apache#23729 from HyukjinKwon/followup-coverage.

Lead-authored-by: Hyukjin Kwon <[email protected]>
Co-authored-by: shane knapp <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: HyukjinKwon

dev/run-tests.py

@@ -469,6 +469,8 @@ def post_python_tests_results():
         run_cmd(["git", "push", "-f", "origin", "gh-pages"])
     finally:
         os.chdir("..")
+        # 10. Remove the cloned repository.
+        shutil.rmtree("pyspark-coverage-site")
 
 
 def run_python_packaging_tests():