gdm: mute debug messages

gdm: mute debug messages

Since we're just using a cut-and-paste of gdm code there's a
slight impedance mismatch between how that code and the rest of
the shell manage debug messages.  In GDM, they're out of sight by
default, but in gnome-shell they're visible and quite verbose.
This muddies up and masks the useful messages that other parts
of the code generate.

This commit just mutes the messages unconditionally by default,
since they aren't that useful anyway.