[media] dvb/audio.h: Remove definition for AUDIO_GET_PTS

While this ioctl is defined inside dvb/audio.h, it is not docummented
at the API specs, nor implemented on any driver inside the Linux Kernel.
So, it doesn't make sense to keep it here.

As this is not used anywere, removing it is not a regression. So,
there's no need to use the normal features-to-be-removed process.

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