mtd: Remove duplicate checks on mtd_oob_ops parameter

Some of the check done in custom ->_read/write_oob() implementation are
already done by the core (in mtd_check_oob_ops()).

Suggested-by: Peter Pan <peterpansjtu@gmail.com>
[Remove redundant checks done in mtdpart.c]
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Ladislav Michl <ladis@linux-mips.org>