mtd: ubi: Remove select for non existent option

There is no 'config CRC32' remove the select that was attempting to use
it.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>