V4L/DVB (10904): remove dib0070_ctrl_agc_filter from dib0070.h

This function prototype is defined but the function itself does not exist.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>