Revert "prefs: Scale the root window cursor by the scale factor"

This reverts commit 4fe66ce0a950134da20089319d68ba679be13d35.

This is wrong ... we should not scale the cursor size but read
the cursor xsettings that gets exported by gsd. Also this won't update on
resolution changes.

https://bugzilla.gnome.org/show_bug.cgi?id=729337
prefs: Update cursor size based on xsettings

We shouldn't scale the cursor size in mutter we g-s-d exports
the correct size on hidpi so use gtk-cursor-theme-size.

This way we also catch changes on resolution updates.

https://bugzilla.gnome.org/show_bug.cgi?id=729337