Remove full_path_project_search feature flag

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR removes the full_path_project_search feature flag.

Rollout issue: #388473 (closed)

Background

!108906 (merged) added a improvement to project searches where namespaces would be included in the search if the query contained /. This change was introduced behind a feature flag so that it could quickly be turned off in case of performance issues. The feature flag was globally enabled on January 31st. For the next 16 hours afterwards, there were no noticeable changes to search duration: #388473 (comment 1259175126). The feature flag has remained enabled for the past month and has been stable. It was then enabled by default on March 9th with !111808 (merged).

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