[media] demux.h: Remove duplicated enum

"enum dmx_ts_pes" and "typedef enum dmx_pes_type_t" are just the
same enum declared twice, since Kernel (2.6.12). There's no reason
to duplicate it there, and sparse complains about that:
	drivers/media/dvb-core/dmxdev.c:600:55: warning: mixing different enum types
So, remove the internal define, keeping just the external one.
Internally, use only "enum dmx_ts_pes", as it is too late to drop
dmx_pes_type_t from the userspace API.

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