Remove foreign key constraint ci_pending_builds.namespace_id

What does this MR do and why?

The loose foreign key was already added for this in !77636 (merged) . We've confirmed in #348269 (closed) that it is indeed safe to swap these foreign keys for loose foreign keys.

Migrations

Up

== 20220106233459 RemoveForeignKeyCiPendingBuildsNamespaceId: migrating =======
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:ci_pending_builds)
   -> 0.0032s
-- remove_foreign_key(:ci_pending_builds, :namespaces, {:name=>"fk_fdc0137e4a"})
   -> 0.0041s
== 20220106233459 RemoveForeignKeyCiPendingBuildsNamespaceId: migrated (0.0179s)

Down

== 20220106233459 RemoveForeignKeyCiPendingBuildsNamespaceId: reverting =======
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:ci_pending_builds)
   -> 0.0035s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE ci_pending_builds\nADD CONSTRAINT fk_fdc0137e4a\nFOREIGN KEY (namespace_id)\nREFERENCES namespaces (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0020s
-- execute("ALTER TABLE ci_pending_builds VALIDATE CONSTRAINT fk_fdc0137e4a;")
   -> 0.0049s
== 20220106233459 RemoveForeignKeyCiPendingBuildsNamespaceId: reverted (0.0270s)

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Related to #348269 (closed)