netfs: Remove unused functions

set_page_fscache(), wait_on_page_fscache() and
wait_on_page_fscache_killable() have no more users.  Remove them and
update the documentation to describe their folio equivalents.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: David Howells <dhowells@redhat.com>