Remove codequality_mr_diff_annotations flag

What does this MR do?

For #330909 (closed)

This MR removes the feature flag :codequality_mr_diff_annotations, which was one of the feature flags involved in the rollout of the inline code quality feature we added to the MR diff. This flag has been enabled on GitLab.com since 2021-06-16 and the feature has behaved as expected since we addressed this issue, so it should be safe to remove the flag now.

Screenshots or Screencasts (strongly suggested)

No visual changes are expected, but this is the feature that the flag controlled:

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?) Documentation was updated when we removed this related feature flag
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?) This is a GitLab Ultimate feature, so the changelog should be in EE even though some of the other file changes are not in ee/
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.