mtd: remove MEMSETOOBSEL macro definition

MEMSETOOBSEL is completely unused and useless. Remove the definition.

Note: it's probably best not to use this ioctl number in the future for
MTD, since that may cause conflicts between old kernels and new user
software (or new kernels and old user software).

Artem: leave a comment about MEMSETOOBSEL.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>