Remove paginated_issue_discussions FF

What does this MR do and why?

Removes the paginated_issue_discussions feature flag.

This has already been enabled for about a month on GitLab.com.

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 #345351 (closed)