media: av7110: remove old documentation files

The documentation has been update and is now at:
linux/Documentation/userspace-api/media/dvb/legacy_dvb_osd.rst
linux/Documentation/userspace-api/media/dvb/legacy_dvb_audio.rst
linux/Documentation/userspace-api/media/dvb/legacy_dvb_video.rst

The .rst files of the old documentation aren't necessary any more,
remove them.

Signed-off-by: Stefan Herdler <herdler@nurfuerspam.de>

Author: Stefan Herdler