mm: remove zero_user()

mm: remove zero_user()

All users have now been converted to either memzero_page() or
folio_zero_range().

Link: https://lkml.kernel.org/r/20250612143443.2848197-6-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Alex Markuze <amarkuze@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Xiubo Li <xiubli@redhat.com>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>