Remove stickyMonitor from diff_file_head.vue

What does this MR do?

Related to: #35814 (closed)

Remove stickyMonitor from diff_file_head.vue

In this scenario, stickyMonitor only adds the class is-stuck to diff_file_header. On large diffs, this almost doubles the render time.

All the is-stuck class does is set border-radius to 0. I believe the performance impact is not worth the trade-off.

Screenshots

The visual difference in changes tabs.

Load difference ~40% reduction in time

Testing

If testing with local seeded projects use this one: http://localhost:3000/Commit451/lab-coat/merge_requests/2/diffs

That is the one I used for my testing.

Here are the stats for that project at the time of writing this MR

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides