Remove issues_full_text_search feature flag

What does this MR do and why?

Remove issues_full_text_search feature flag

This has been enabled by default since 15.2

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 #354784 (closed)