refactor(queueworker): remove unused function and import

Remove `_close_db_handle` method from `QueueWorker` class. Nowhere
calls this method, so it is safe to remove. This function was the
only place using the `db` imported from `data.model`, so we can
remove that import as well.

Testing: need to look into it

workers/queueworker.py

@@ -4,7 +4,6 @@ import json
 from threading import Event, Lock
 
 from app import app
-from data.model import db
 from data.database import CloseForLongOperation
 from workers.worker import Worker
 
@@ -57,11 +56,6 @@ class QueueWorker(Worker):
     """ Function that gets run once every watchdog_period_seconds. """
     pass
 
-  def _close_db_handle(self):
-    if not db.is_closed():
-      logger.debug('Disconnecting from database.')
-      db.close()
-
   def extend_processing(self, seconds_from_now, updated_data=None):
     with self._current_item_lock:
       if self.current_queue_item is not None: