mm: remove page_mapping_file()

This function has no more users.

Link: https://lkml.kernel.org/r/20230802151406.3735276-31-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>