[Fix] Remove the db at the end of PaimonRWHandleTest

**Describe the bug**
After running PaimonRWHandleTest, the directory of the db is not removed, which would cause some error when running the same UT next time.

**Expected behavior**
After running PaimonRWHandleTest, the directory of the db should be removed.

**Additional context**
Add any other context about the problem here.
