port to libsoup3/remove proxy settings

https://salsa.debian.org/mpd-team/gmpc/-/blob/debian/latest/debian/patches/download-Remove-proxy-settings.patch
Index: src/internal-plugins.h
--- src/internal-plugins.h.orig
+++ src/internal-plugins.h
@@ -5,7 +5,6 @@ extern gmpcPlugin connection_plug;
 extern gmpcPlugin playlist_plug;
 extern gmpcPlugin cover_art_plug;
 extern gmpcPlugin tray_icon2_plug;
-extern gmpcPlugin proxyplug;
 extern gmpcPlugin playlist_editor_plugin;
 extern gmpcPlugin statistics_plugin;
 extern gmpcPlugin extraplaylist_plugin;
