x86/uaccess: Remove memcpy_page_flushcache()

Commit 21b56c84 ("iov_iter: get rid of separate bvec and xarray
callbacks") removed the calls to memcpy_page_flushcache().

In addition, memcpy_page_flushcache() uses the deprecated
kmap_atomic().

Remove the unused x86 memcpy_page_flushcache() implementation and
also get rid of one more kmap_atomic() user.

[ dhansen: tweak changelog ]

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20221230-kmap-x86-v1-1-15f1ecccab50%40intel.com