mm/ksm: remove redundant code in ksm_fork

mm/ksm: remove redundant code in ksm_fork

Since commit 3c6f33b7273a ("mm/ksm: support fork/exec for prctl"), when a
child process is forked, the MMF_VM_MERGE_ANY flag will be inherited in
mm_init(). So, it's unnecessary to set the flag in ksm_fork().

Link: https://lkml.kernel.org/r/20240402024934.1093361-1-tujinjiang@huawei.com
Signed-off-by: Jinjiang Tu <tujinjiang@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Nanyong Sun <sunnanyong@huawei.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Stefan Roesch <shr@devkernel.io>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>