models: Don't require test coverage for clear_database.

This function is only used in populate_db, not at runtime, and is
inefficient to test, since it's main role is deleting everything in
the database.

zerver/models.py

@@ -1544,7 +1544,7 @@ def get_huddle_backend(huddle_hash: Text, id_list: List[int]) -> Huddle:
             Subscription.objects.bulk_create(subs_to_create)
         return huddle
 
-def clear_database() -> None:
+def clear_database() -> None:  # nocoverage # Only used in populate_db
     pylibmc.Client(['127.0.0.1']).flush_all()
     model = None  # type: Any
     for model in [Message, Stream, UserProfile, Recipient,