seat: remove wlr_seat_validate_grab_serial()
It makes little sense to have a catch-all grab vaildation function, considering that e.g. tablet tool implicit grabs are possible as well. Besides, the function has always returned true anyway.
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It makes little sense to have a catch-all grab vaildation function, considering that e.g. tablet tool implicit grabs are possible as well. Besides, the function has always returned true anyway.