Remove the suggestion_highlight_uses_note_diff_file feature flag

Removes the suggestion_highlight_uses_note_diff_file feature flag. The flag has been at 100% on GitLab.com since 2026-09-24 with no attributed exceptions, so the flag-on path (highlighting suggestions from the stored diff file instead of a Gitaly compare per discussion) is now the only path.

Resolves #629414 (closed)

Detailed context for AI agents

Flag

• suggestion_highlight_uses_note_diff_file, type gitlab_com_derisk, group::code review, introduced in 19.5 by !255964 (merged).
• Feature issue: https://gitlab.com/gitlab-com/gl-infra/production/-/issues/22925
• Rollout issue: #629414 (closed)

Why the flag existed

• A production incident where gitlab-org/gitlab hit a Gitaly CommitDiff rate limit on the merge request discussions endpoint.

What the flag gated

• Suggestion#diff_file_for_highlight in app/models/suggestion.rb.
• Flag on: returns note.discussion.diff_file, which is built from the stored note_diff_files row of the discussion's first note.
• Flag off (old path): returned note.latest_diff_file, which runs a Gitaly compare (one CommitDiff plus two FindCommit calls) per suggestion-bearing discussion on every discussions.json load.

Rollout history on GitLab.com

• Staging and test project: 2026-09-18
• gitlab-org/gitlab: 2026-09-18
• 25% of actors: 2026-09-22
• 50%: 2026-09-23
• 100%: 2026-09-24
• No exceptions were attributed to the flag. CommitDiff volume from the discussions controller dropped as expected.

Changes in this MR (24 lines deleted, nothing added)

• Deleted the flag check line in app/models/suggestion.rb.
• Deleted config/feature_flags/gitlab_com_derisk/suggestion_highlight_uses_note_diff_file.yml.
• Removed the "when suggestion_highlight_uses_note_diff_file is disabled" context from spec/models/suggestion_spec.rb.
• Suggestion#diff_file is kept because outdated? and the apply path still use it.
• The commit has Changelog: performance.

Behavioural notes (unchanged from the original MR)

• Discussions whose first note has no stored note_diff_files row still fall back to the Gitaly compare inside DiffDiscussion#diff_file.
• Cosmetic differences are limited to syntax highlighting of suggestion previews:
  • The lexer follows the file path at comment time, so a later rename that changes the extension picks a different lexer.
  • The "too large to highlight" check uses blob sizes at the original refs rather than head.
• Applying suggestions is unchanged.

Verification

• bundle exec rspec spec/models/suggestion_spec.rb - 36 examples, 0 failures.
• Rubocop clean on both changed Ruby files.