Clean up backward compatibility codes for !82801

For !82801 (comment 873435160)

In the above merge request, we limit the number of commits displayed in the emails triggered when there is a push to a merge request. To achieve the above task, we modified the existing mailer Emails:: MergeRequests#push_to_merge_request_email. This method was added with 2 new parameters total_new_commits and total_existing_commits. The default values are nil. To guarantee the backward compatibility of the mailer jobs, we added some codes to fulfill the missing arguments of prior mailer. Those lines of codes should be removed after some releases to avoid confusion later.

This issue acts as a reminder. I'll set a due date to after some months. When the time comes, we'll re-evaluate the situation to see whether it's a good time for such removal.