Remove quota notifications from gsd-plugin

[Alejandro J. Cura]

The out-of-quota notifications for the users' folders and for shared folders are being shown as a small dialog from the gnome-settings-daemon plugin (in addition to the new feature of bouncing the launcher icon and showing that error on the control panel.)

Since we don't want to have both working at once, the dialog shown from the g-s-d plugin should be removed.