xsettings: always use mutter-computed UI scaling value
requested to merge 3v1n0/gnome-settings-daemon:xsettings-use-mutter-legacy-scaling-factor into master
Mutter already computes the proper window scaling level to be used under X11, this logic is already present in mutter, so there's no point to change it here and go into the implementations details wich should be private at this level. So just use the 'legacy-ui-scaling-factor' key value that we can trust.