Remove `issue_autocomplete_backend_filtering` feature flag

What does this MR do and why?

Remove issue_autocomplete_backend_filtering feature flag to filter issue autocomplete results on the backend to improve performance for projects with a lot of issues.

This has been enabled on GitLab.com for almost 2 weeks. For gitlab-org/gitlab, this improves response time from ~15 seconds to <1 second.

Related to #289238 (closed) #488831 (closed)

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