nandwrite: kill -j, -y, and -f options

nandwrite: kill -j, -y, and -f options

The legacy -j (--jffs2) and -y (--yaffs) options haven't been
operational for a long time, since MEMSETOOBSEL was killed. I don't
think anybody will miss these options (correct me if I'm wrong). They
can be replaced by proper usage of MTD_OOB_AUTO modes.

The -f (--forcelegacy) option went hand in hand with -j and -y. Now that
-j and -y are gone, there's no use for -f. Kill it.

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