media: cec: remove unused functions

Remove several functions that are no longer used now that the
conversion of cec drivers to cec_notifier_conn_(un)register() and
cec_notifier_cec_adap_(un)register() is complete.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>