Remove a statement using a name that is never defined in scope.

The intention may have been to close the “current” database cursor; but
there is no reference to that at this point in the code.

auth-mastodon.py

@@ -69,7 +69,6 @@ except Exception:
 
 @atexit.register
 def close_db():
-    cursor.close()
     database.close()