mm: compaction: remove unneeded assignment to isolate_start_pfn

isolate_start_pfn is unused when cc->nr_freepages !  = 0.  Otherwise
cc->free_pfn will overwrite it unconditionally.  So we should remove this
unneeded and somewhat misleading assignment.

Link: https://lkml.kernel.org/r/20220418141253.24298-4-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Charan Teja Kalla <charante@codeaurora.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Pintu Kumar <pintu@codeaurora.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>