Fix AppVeyor build with Gtk
Fix AppVeyor build with Gtk

Do not use the broken Gtk Chocolatey package, download Gtk packge
directly from Xamarin instead.

DotNetDoctor/oxyplot,svendu/oxyplot,shoelzer/oxyplot,jeremyiverson/oxyplot,svendu/oxyplot,jeremyiverson/oxyplot,objorke/oxyplot,as-zhuravlev/oxyplot_wpf_fork,bbqchickenrobot/oxyplot,objorke/oxyplot,shoelzer/oxyplot,bbqchickenrobot/oxyplot,jeremyiverson/oxyplot,br111an/oxyplot,shoelzer/oxyplot,Rustemt/oxyplot,Mitch-Connor/oxyplot,Rustemt/oxyplot,H2ONaCl/oxyplot,as-zhuravlev/oxyplot_wpf_fork,ze-pequeno/oxyplot,Kaplas80/oxyplot,Sbosanquet/oxyplot,br111an/oxyplot,DotNetDoctor/oxyplot,olegtarasov/oxyplot,bbqchickenrobot/oxyplot,Sbosanquet/oxyplot,GeertvanHorrik/oxyplot,zur003/oxyplot,Mitch-Connor/oxyplot,GeertvanHorrik/oxyplot,olegtarasov/oxyplot,Jonarw/oxyplot,TheAlmightyBob/oxyplot,as-zhuravlev/oxyplot_wpf_fork,HermanEldering/oxyplot,freudenthal/oxyplot,Sbosanquet/oxyplot,Mitch-Connor/oxyplot,Rustemt/oxyplot,TheAlmightyBob/oxyplot,H2ONaCl/oxyplot,svendu/oxyplot,freudenthal/oxyplot,oxyplot/oxyplot,Kaplas80/oxyplot,Isolocis/oxyplot,NilesDavis/oxyplot,H2ONaCl/oxyplot,objorke/oxyplot,GeertvanHorrik/oxyplot,br111an/oxyplot,TheAlmightyBob/oxyplot,HermanEldering/oxyplot,freudenthal/oxyplot,Jofta/oxyplot