Remove async_merge_request_check_mergeability feature flag

What

The async_merge_request_check_mergeability will be enabled by default in 13.0 but the feature flag wont be removed just in case there are issues that come up because of it.

This issue is for removing the flag entirely in a future release (hopefully 13.1).

Owners

• Team: Create::Source Code
• Most appropriate slack channel to reach out to: #g_create_source_code
• Best individual to reach out to: @patrickbajao

Expectations

Roll Out Steps

• Remove feature flag and add changelog entry
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel