mm: remove total_mapcount()

All users of total_mapcount() are gone, let's remove it.

Link: https://lkml.kernel.org/r/20240226141324.278526-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>