Documentation/admin-guide/mm/pagemap.rst: drop "Using pagemap to do something useful"

Documentation/admin-guide/mm/pagemap.rst: drop "Using pagemap to do something useful"

That example was added in 2008.  In 2015, we restricted access to the PFNs
in the pagemap to CAP_SYS_ADMIN, making that approach quite less usable.

It's 2024 now, and using that racy and low-lewel mechanism to calculate
the USS should not be considered a good example anymore.  /proc/$pid/smaps
and /proc/$pid/smaps_rollup can do a much better job without any of that
low-level handling.

Let's just drop that example.

Link: https://lkml.kernel.org/r/20240607122357.115423-7-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Oscar Salvador <osalvador@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>