Revert: Validate foreign key on GroupHooks

What does this MR do?

The change in !60266 (merged) was not good. There was (at least) one invalid row in web_hooks:

gitlabhq_production=> select id from web_hooks where group_id = 2276106;
   id   
--------
 267051
(1 row)

gitlabhq_production=> select id from namespaces where id = 2276106;
 id 
----
(0 rows)

As detected by the database migrations pipeline

Does this MR meet the acceptance criteria?

Conformity

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