Move MergeRequests::CreateService#after_create items to MergeRequests::AfterCreateService

MergeRequests::CreateService#after_create runs in the request where an MR is created.

MergeRequests::AfterCreateService runs in Sidekiq after an MR is created.

Ideally, the former would be empty - or limited to things that are essential to happen in the creation HTTP request.

In !26172 (merged) I moved pipeline creation to Sidekiq, but I left the remaining items. We should check those and move them if possible.