spice-display: Remove auto-usbredirection option

Since this option is not the most obvious one and given that we refresh
properties view when new devices are plugged-in and users have easy way
to redirect devices individually, lets just remove this option.

I told jimmac I'm going to do this and his reply was "\o/". :)

https://bugzilla.gnome.org/show_bug.cgi?id=720792