Remove unused playground applets and engines: cloud, jssample, jsengine,...

Remove unused playground applets and engines: cloud, jssample, jsengine, lastfmevents, scriptedsample, video, wiki

These weren't built even with WITH_PLAYGROUND=ON. Amarok code is not
a place to dump half-working code snippets. Individual applets and engines
can be revived if it is found they are of any value.

CCMAIL: amarok-devel@kde.org