Delete the add_organization_to_diff_commit_users feature flag

Rollout issue: #546221 (closed)

Currently the feature flag is enabled on production

TODO for the reviewers before merging this MR

• See the status of the rollout by checking #546221 (closed), https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues/?search=add_organization_to_diff_commit_users&sort=created_date&state=all&label_name%5B%5D=host%3A%3Agitlab.com
• Verify the feature flag status via chatops by running /chatops run feature get add_organization_to_diff_commit_users.
• Search for references to AddOrganizationToDiffCommitUsers in frontend part of code
• Search for references to add_organization_to_diff_commit_users in code
• Check if we need to remove any Gem or other related code by looking at the changes in !186458 (merged)

This change was generated by gitlab-housekeeper locally using the Keeps::DeleteOldFeatureFlags keep.

To provide feedback on your experience with gitlab-housekeeper please create an issue with the label GitLab Housekeeper and consider pinging the author of this keep.