Remove feature flag: policy_mergability_check

What does this MR do and why?

For #535271 (closed)

Removes the policy_mergability_check feature flag, which has been default on for a while.

In addition to removing the flag, also removes code blocks guarded by the flag.

Screenshots or screen recordings

All backstage. All code removed should have been inert/skipped anyway, so no functionality should change at all.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.