mm/hmm.c: remove set but not used variable 'devmem'

Fixes gcc '-Wunused-but-set-variable' warning:

mm/hmm.c: In function 'hmm_devmem_ref_kill':
mm/hmm.c:995:21: warning:
 variable 'devmem' set but not used [-Wunused-but-set-variable]

It not used any more since 35d39f953d4e ("mm, hmm: replace
hmm_devmem_pages_create() with devm_memremap_pages()")

Link: http://lkml.kernel.org/r/1543629971-128377-1-git-send-email-yuehaibing@huawei.comSigned-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hmm.c

@@ -992,9 +992,6 @@ static void hmm_devmem_ref_exit(void *data)
 
 static void hmm_devmem_ref_kill(struct percpu_ref *ref)
 {
-	struct hmm_devmem *devmem;
-
-	devmem = container_of(ref, struct hmm_devmem, ref);
 	percpu_ref_kill(ref);
 }