[media] dvb: Remove unused frontend sources at demux.h and sync doc

The DVB core has a provision for other frontend sources, but no
drivers use it. The kdapi.xml contains provision for some other
frontend source types, but it is not in sync with the code.

So, remove the unused types and sync both files.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>