[media] DVB-CORE: remove superfluous DTV_CMDs

This small patch removes superfluous DTV_CMDs from dvb_frontend.c which were added in the initially when ISBD-T support was added.
They were there unnoticed even though compilers should have warning about those duplicates. Finally they did and now we can remove them.

Thanks to Dan Carpenter <dan.carpenter@oracle.com> for pointing that out.

Signed-off-by: Patrick Boettcher <pboettcher@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>