mm/percpu: remove unused pcpu_map_extend_chunks

Since commit 40064aec ("percpu: replace area map allocator with
bitmap"), it is unneeded.
Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

mm/percpu.c

@@ -174,9 +174,6 @@ static DEFINE_MUTEX(pcpu_alloc_mutex);	/* chunk create/destroy, [de]pop, map ext
 
 struct list_head *pcpu_chunk_lists __ro_after_init; /* chunk list slots */
 
-/* chunks which need their map areas extended, protected by pcpu_lock */
-static LIST_HEAD(pcpu_map_extend_chunks);
-
 /*
  * The number of empty populated pages, protected by pcpu_lock.
  * The reserved chunk doesn't contribute to the count.