Remove command palette POC (#24044)

* Revert "POC: Cross-team recent search (#20027)"

This reverts commit aa59c28b04, preserving
a few code tidyings unrelated to the original PR.

* Revert "Add feature flag for command palette (#20011)"

This reverts commit c78c5ce3f3.

server/channels/db/migrations/mysql/000084_recent_searches.down.sql

@@ -1 +1,2 @@
-DROP TABLE IF EXISTS RecentSearches;
+-- This table is unused, and will be dropped in a future ESR.
+DROP TABLE IF EXISTS RecentSearches;