Remove update merge request related feature flags [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Removes the following feature flags:

• async_handle_merge_request_assignees_change
• resolve_merge_request_todos_async

These flags were enabled for everyone on GitLab.com for 24 hours and the workers are performing and working fine.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)

Related to #20827 (closed)