nand: yaffs: Remove the "nand write.yaffs" command

This command is only enabled by one board, complicates the NAND code,
and doesn't appear to have been functioning properly for several
years.  If there are no bad blocks in the NAND region being written
nand_write_skip_bad() will take the shortcut of calling nand_write()
which bypasses the special yaffs handling.  This causes invalid YAFFS
data to be written. See
http://lists.denx.de/pipermail/u-boot/2011-September/102830.html

 for
an example and a potential workaround.

U-Boot still retains the ability to mount and access YAFFS partitions
via CONFIG_YAFFS2.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>