gtk3-demo: Drop unneeded gtk_init() call
gtk3-demo: Drop unneeded gtk_init() call

GtkApplication is doing this for us.

jessevdk/gtk,jigpu/gtk,jigpu/gtk,Sidnioulz/SandboxGtk,chergert/gtk,ahodesuka/gtk,davidgumberg/gtk,jadahl/gtk,jigpu/gtk,Lyude/gtk-,alexlarsson/gtk,grubersjoe/adwaita,Adamovskiy/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,ahodesuka/gtk,jigpu/gtk,jigpu/gtk,davidgumberg/gtk,jigpu/gtk,jessevdk/gtk,chergert/gtk,Adamovskiy/gtk,Lyude/gtk-,Sidnioulz/SandboxGtk,Lyude/gtk-,jadahl/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,jadahl/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,davidgumberg/gtk,alexlarsson/gtk,jadahl/gtk,Lyude/gtk-,davidgumberg/gtk,jessevdk/gtk,Adamovskiy/gtk,Lyude/gtk-,grubersjoe/adwaita,chergert/gtk,jadahl/gtk,ahodesuka/gtk,jadahl/gtk,grubersjoe/adwaita,jadahl/gtk,grubersjoe/adwaita,ahodesuka/gtk,Lyude/gtk-,alexlarsson/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jessevdk/gtk,ahodesuka/gtk,Adamovskiy/gtk,jigpu/gtk,ahodesuka/gtk,Sidnioulz/SandboxGtk,chergert/gtk,jessevdk/gtk,jadahl/gtk,alexlarsson/gtk,jigpu/gtk,jessevdk/gtk,davidgumberg/gtk,alexlarsson/gtk,grubersjoe/adwaita,davidgumberg/gtk,davidgumberg/gtk,chergert/gtk,jessevdk/gtk,ahodesuka/gtk,grubersjoe/adwaita,Adamovskiy/gtk,chergert/gtk,Lyude/gtk-,chergert/gtk,chergert/gtk,grubersjoe/adwaita,davidgumberg/gtk,grubersjoe/adwaita,Lyude/gtk-,Adamovskiy/gtk,ahodesuka/gtk