mtd: mtdoops: kill Kconfig usage instructions

mtd: mtdoops: kill Kconfig usage instructions

The mtdoops usage instructions found in Kconfig have been incorrect
since:

    commit 2e386e4bac90554887e73d6f342e845185b33fc3
    mtd: mtdoops: refactor as a kmsg_dumper

mtdoops no longer uses a console. Now, if you build it into your kernel,
you add something like the following to your command line to select
partition X as your logging partition:

    mtdoops.mtddev=X

Anyway, it seems better to leave the documentation out of Kconfig.

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