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/Tools/plugin-man.c
--- src/Tools/plugin-man.c.orig
+++ src/Tools/plugin-man.c
@@ -74,7 +74,6 @@ void plugin_manager_load_internal_plugins(void)
     plugin_add(&playlist_editor_plugin, 0, NULL);
 
     plugin_add(&statistics_plugin, 0, NULL);
-    plugin_add(&proxyplug, 0, NULL);
 
     plugin_add(&extraplaylist_plugin, 0, NULL);
     plugin_add_new(
