mm: remove page_has_private()

This function has no more callers, except folio_has_private().  Combine
the two functions.

Link: https://lkml.kernel.org/r/20240821193445.2294269-9-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>