Remove foreign key constraint ci_runner_namespaces.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 this foreign key for a loose foreign key.

Migrations

Up

== 20220106235626 RemoveForeignKeyCiRunnerNamespacesNamespaceId: migrating ====
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:ci_runner_namespaces)
   -> 0.0030s
-- remove_foreign_key(:ci_runner_namespaces, :namespaces, {:name=>"fk_rails_f9d9ed3308"})
   -> 0.0054s
== 20220106235626 RemoveForeignKeyCiRunnerNamespacesNamespaceId: migrated (0.0178s)

Down

== 20220106235626 RemoveForeignKeyCiRunnerNamespacesNamespaceId: reverting ====
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:ci_runner_namespaces)
   -> 0.0031s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE ci_runner_namespaces\nADD CONSTRAINT fk_rails_f9d9ed3308\nFOREIGN KEY (namespace_id)\nREFERENCES namespaces (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0023s
-- execute("ALTER TABLE ci_runner_namespaces VALIDATE CONSTRAINT fk_rails_f9d9ed3308;")
   -> 0.0033s
== 20220106235626 RemoveForeignKeyCiRunnerNamespacesNamespaceId: reverted (0.0169s)

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Related to #348269 (closed)