docs: cgroup-v1: Update page cache removal functions

Commit 452e9e69 ("filemap: Add filemap_remove_folio and
__filemap_remove_folio") reimplemented __delete_from_page_cache() as
__filemap_remove_folio() and delete_from_page_cache() as
filemap_remove_folio().  The compatibility wrappers were finally removed
in ece62684 ("hugetlbfs: convert hugetlb_delete_from_page_cache() to
use folios") and 6ffcd825 ("mm: Remove __delete_from_page_cache()").

Update the remaining references to dead functions in the memcg
implementation memo.

Signed-off-by: Illia Ostapyshyn <illia@yshyn.com>
Signed-off-by: Tejun Heo <tj@kernel.org>