mm/memory-failure: remove MF_MSG_SLAB

mm/memory-failure: remove MF_MSG_SLAB

Since commit 46df8e73a4a3 ("mm: free up PG_slab"), MF_MSG_SLAB becomes
unused.  Remove it.  No functional change intended.

Link: https://lkml.kernel.org/r/20240612071835.157004-3-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: David Hildenbrand <david@redhat.com>
Cc: kernel test robot <lkp@intel.com>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>