Delete all references to static site editor feature flags after removal is deployed

After Remove all SSE references [RUN ALL RSPEC] [RUN ... (!87221 - merged) is deployed to production, we need to clean up the related feature flags which have been removed:

• config/feature_flags/development/usage_data_static_site_editor_commits.yml
• config/feature_flags/development/usage_data_static_site_editor_merge_requests.yml

Feature flags were removed separately as part of Remove dynamic check for feature flag (!86913 - merged) (merge commit f39cc25c439a029430e741d8e625d0e90aea7de2)

Tasks

Clean up the feature flag from all environments by running these chatops command in #production channel:

• Verify removal has been deployed everywhere: /chatops run auto_deploy status f39cc25c439a029430e741d8e625d0e90aea7de2
• /chatops run feature delete <FLAG_NAME> --dev
• /chatops run feature delete <FLAG_NAME> --staging
• /chatops run feature delete <FLAG_NAME>