mutter-launch: simplify by removing features we don't need

Remove the ability to launch as a different user, which we don't
need because we're spawned by gdm or by the user manually on the
command line.
At the same time, require an active local session, and remove
the ability to run from anywhere by being in the right user group
(which automatically gives you root-like privileges)

https://bugzilla.gnome.org/show_bug.cgi?id=705861