Remove rate_limit_issuable_searches feature flag

What does this MR do and why?

Removes the rate_limit_issuable_searches feature flag. This has already been enabled on GitLab.com for a week.

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 #386544 (closed)