arch/um/kernel/ptrace.c: remove dead PROC_MM

PROC_MM doesn't exist in Kconfig.  Looking around it looks like a
left-over from 2.6.0 or even 2.4 times, last mentioned in a fedora patch
for 2.6.10.  I believe it's time to get rid of that last tiny parts here
that are still around.

Signed-off-by: Christoph Egger <siccegge@cs.fau.de>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>