agp: remove read/write stubs

The VFS layer handles those in the very same way, if unset. No need for
additional stubs.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>