[After %15.2] Remove wip/unwip from app/services/merge_requests/base_service.rb handle_draft_event

This is a follow up to #363403 (closed) and !89271 (merged) to be worked on after %15.2!

In !89271 (merged), we updated MergeRequests::BaseService#handle_draft_event to accept wip/unwip and draft/ready as internal draft event states. This should ensure a smooth transition from the former event states wip/unwip to the more recent draft/ready states, as the states are used in frontend JavaScript app/assets/javascripts/merge_request.js.

As discussed in !89271 (merged), after %15.2, we can remove the former event states wip/unwip from [MergeRequests::BaseService#handle_draft_event](app/services/merge_requests/base_service.rb#L134] and the accompanying code comment (while keeping event states draft/ready).

Implementation Guide

After %15.2:

• Open app/services/merge_requests/base_service.rb
• Navigate to method MergeRequests::BaseService#handle_draft_event
• Remove line when 'wip' then MergeRequest.draft_title(title)
• Remove line when 'unwip' then MergeRequest.draftless_title(title)
• Remove comment lines Supports both wip and draft permutations of draft_event., This support can be removed >= %15.2