Revert "Add Compiz session."

This reverts commit 09078e79b2cc56a95652f6bff3824f2357c2ee95.

Compiz has nothing to do with Gnome Flashback (only Gnome panel),
we don't support Compiz upstream, and probably never will.

Bugzilla was: https://bugzilla.gnome.org/show_bug.cgi?id=719424
Revert "Add Mutter session."

This reverts commit 8ce8bf9707815df7d3f186be669c22911375ea08.

It is not possible with all DMs to list parameters in the session file
this way. :-(

Also we need to discuss supporting a mutter configuration first.

Bugwilla was: https://bugzilla.gnome.org/show_bug.cgi?id=719424