Remove 'comment_on_any_diff_line' feature flag

Oswaldo Ferreirarequested to merge
osw-remove-comment-on-any-diff-line-ff into master

What does this MR do?

Removes the feature flag being used by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22914. Ideally, this should be merged before / on the 20th.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54034

Edited by Oswaldo Ferreira

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