fix_unreads: Remove commit() call in fix().

The commit() call in fix() breaks migrations and tests (unless you
mock) due to outer transactions.

We now explicitly call commit() from the management command.

zerver/lib/fix_unreads.py

@@ -248,4 +248,3 @@ def fix(user_profile):
     with connection.cursor() as cursor:
         fix_unsubscribed(cursor, user_profile)
         fix_pre_pointer(cursor, user_profile)
-        connection.commit()