[Improvement] dropTable unexpectedly uses purgeTable in GravitinoPaimonCatalog.java

### What would you like to be improved?

@caican00 can you confirm if this is intentional?

### How should we improve?

_No response_

Start by reading GravitinoPaimonCatalog.java and trace the implementations or call sites for dropTable and purgeTable. Determine whether their different table-removal semantics are intentional; the issue is done only when that behavior is confirmed and the expected outcome is documented or covered by the relevant project tests.

Written by the indexing model from the issue text.