V4L/DVB (3727): Remove DMX_GET_EVENT and associated data structures

V4L/DVB (3727): Remove DMX_GET_EVENT and associated data structures

The ioctl DMX_GET_EVENT has never been implemented.
I guess no software is using it because of its lack of implementation.
Future software won't use it, too, because this API doesn't make much
sense the way it is: Frontend events have their own different API.
Scrambling events can't be generated in a useful way by the hardware I
know of.

Signed-off-by: Andreas Oberritter <obi@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>