Remove post_import_repository_size_check feature flag

What does this MR do and why?

Removes the post_import_repository_size_check that was introduced in %16.1 via https://gitlab.com/gitlab-org/gitlab/-/issues/395764.

It's been enabled on .com since June.

Screenshots or screen recordings

N/A

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #414530 (closed)