libre/iceweasel: refactor some prefs overrides
    
    Preference overrides from vendor.js are not connected to the code, so
    they tend to bit rot quite quickly. Also, they only hide the problem,
    because the source code still contains references to nonfree stuff.
    
    Patching has a downside too. If upstream adds an override for some
    patched preference in a new place, it's likely to take precedence over
    the patch. Solution for this problem is TBD. But there are ways to
    override our vendor.js too, so either approach is prone to this.
    
    For now, go on with the patching approach. Also drop the override for
    ui.key.menuAccessKeyFocuses as I don't see how this is related to
    freedom issues. Let's stick as close to upstream as possible.
    
    geo.wifi.uri was probably removed upstream as I wasn't able to find it
    anywhere in the source tree.