Revert "Only check valid changes during diff checks"

Purpose of revert

I've merged an MR to set merge_commit_diff_modes FF to enabled by default, however, we have received a QA report of a potential regression that needs investigation Failure in ee/browser_ui/3_create/repository/fi... (#422062 - closed)

To prevent an incident we are reverting this change and disabling the flag on all SAAS environments.

Checklist

• Create an issue to reinstate the merge request and assign it to the author of the reverted merge request.
• If the revert is to resolve a broken 'master' incident, please read through the Responsibilities of the Broken master resolution DRI.
• If the revert involves a database migration, please read through Deleting existing migrations.
• Add the appropriate labels before the MR is created. We can skip CI/CD jobs only if the labels are added before the CI/CD pipeline is created.

Milestone info

• I am reverting something in the current milestone. No changelog is needed, and I've added a ~"regression:*" label.
• I am reverting something in a different milestone. A changelog is needed, and I've removed the ~"regression:*" label.

Related issues and merge requests

• Original feature issue: #23625
• Possible regression issue: #422062 (closed)
• MR to add logic wrapped in FF: !123947 (merged)
• MR to set default enabled: !129465 (merged)