mm/percpu.c: remove the lcm code since block size is fixed at page size

Since commit b239f7da ("percpu: set PCPU_BITMAP_BLOCK_SIZE to
PAGE_SIZE"), the PCPU_BITMAP_BLOCK_SIZE has been set to page size
fixedly. So the lcm code in pcpu_alloc_first_chunk() doesn't make
sense any more, clean it up.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>