lib: remove lib/nodemask.c

Commit 36d4b36b ("lib/nodemask: inline next_node_in() and
node_random()") removed the lib/nodemask.c file, but the remove didn't
happen when the patch was applied.

Reported-by: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>