[media] dvb-core: get rid of demux optional circular buffer

There is a provision at the dvb_demux.c to use a vmalloc'ed
circular buffer, enabled via an extra #ifdef option that it
is not at Kconfig. Enabling it will only make the Kernel to
allocate/deallocate such buffer, but no code would actually
use it. So, no practical effect, except for sparing some
memory without any good reason.

So, get rid of such dead code.

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