V4L/DVB (11437): pvrusb2: Drop client_register/unregister stubs

The client_register and client_unregister methods are optional so
there is no point in defining stub ones. Especially when these methods
are likely to be removed soon.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>