mm: remove PageKsm()

All callers have been converted to use folio_test_ksm() or
PageAnonNotKsm(), so we can remove this wrapper.

Link: https://lkml.kernel.org/r/20241002152533.1350629-6-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Alex Shi <alexs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>