mtd: doc: remove support for DoC 2000/2001/2001+

These drivers are deprecated for very long time, and we have a different driver
for these called "diskonchip". Thus, kill the ancient cruft.

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