[Feature flag] Removal/tracking of `global_search_*_tab` ops feature flags

What

Remove the :global_search_*_tab feature flags when the search cluster is stable and we don't need to use them anymore.

Feature flags:

• global_search_code_tab
• global_search_commits_tab
• global_search_issues_tab
• global_search_merge_requests_tab
• global_search_wiki_tab

Owners

• Team: Global Search
• Most appropriate slack channel to reach out to: #g_global_search
• Best individual to reach out to: @dgruzd

Expectations

What are we expecting to happen?

What might happen if this goes wrong?

What can we monitor to detect problems with this?