mtd: spi-nor: drop replaceable wait-till-ready function pointer

We don't need to expose a 'wait-till-ready' interface to drivers. Status
register polling should be handled by the core spi-nor.c library, and as
of now, I see no need to provide a special driver-specific hook for it.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>