Remove Gitlab::Kas.enabled setting

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The following discussion from !215742 (merged) should be addressed:

• @ash2k started a discussion: (+1 comment)

  Unrelated to this MR: we should remove this flag. We have more and more functionality relying on kas so allowing to switch it off doesn't make sense.

Another issue where this setting caused a problem for a customer: https://gitlab.com/gitlab-com/request-for-help/-/issues/3894#note_3022345186