Refactor. (#4487)

### What problem does this PR solve?

### Type of change

- [x] Refactoring

api/db/db_models.py

@@ -1051,11 +1051,6 @@ def migrate_db():
             )
         except Exception:
             pass
-        try:
-            DB.execute_sql('ALTER TABLE llm DROP PRIMARY KEY;')
-            DB.execute_sql('ALTER TABLE llm ADD PRIMARY KEY (llm_name,fid);')
-        except Exception:
-            pass
         try:
             migrate(
                 migrator.add_column('task', 'retry_count', IntegerField(default=0))