nandwrite: kill `--raw' option

nandwrite: kill `--raw' option

The `--raw' option has lost all usefulness as it overlapped with several
other OOB modes. I cannot even figure out what it was actually intended
to do, but I'm sure its functionality fits somewhere in the
MTD_OPS_{AUTO_OOB,PLACE_OOB,RAW} options, which are mostly implemented
in libmtd's mtd_write().

I don't think users need a warning for this one, unless someone can tell
me what it actually was supposed to have done in the first place.

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