Resolve "[Feature flag] Rollout of `local_file_reviews`"

What does this MR do?

For #296674 (closed)

Removes the local_file_reviews feature flag, leaving the new feature code intact.

Screenshots or Screencasts (strongly suggested)

N/A: the feature is unchanged.

How to setup and validate locally (strongly suggested)

1. Create an MR with diffs
2. Ensure you are authenticated to GitLab/the GDK
3. Note that the "Viewed" checkbox for marking a file as reviewed remains on the diffs before (by default, or after explicitly enabling the feature) as well as after (it's not a flagged feature)

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?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• 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.
• This change is backwards compatible across updates, or this does not apply.

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.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.