[voicecall] Remove unused tonegend plugin.

Hasn't been used since c86394a213f810cf545707794816f31265a2f179 (2012!)
[voicecall] don't use QDBusInterface for tonegend

since we're just making a simple method call, go ahead and call it, and avoid
the blocking overhead to set up the interface.
Merge pull request #55 from rburchell/master

tonegend-related cleanup