Remove/Refactor boards code

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On EE, it looks like we don't use NUMBER_OF_PERMITTED_BOARDS or validate_board_limit(board) - which we do use on CE in order to validate the boards.

We should either remove them or refactor them to be meaningful on EE, as at the moment this is not in use.

More details: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1630#note_27347611