Delete the 'slides' processing scripts

The slides processing scripts have never been updated for 0.9.0 events,
and probably never will be. They're still using older video/audio
processing apis that have poor sync, too. Just remove them.

The playback support files will still be installed, to allow recordings
from old BigBlueButton servers to be copied to a 0.9.0 server.
Merge pull request #431 from kepstin/recording-updates

Delete the 'slides' processing scripts