Remove RSS support

RSS support is blocking a few simplifications for some other
improvements, and it additionally is very much untested and relies on
old, abandoned libraries for XML parsing.

Other technical factors influenced this decision (see github issue #191)
but the primary motivation is developer time and knowledge to maintain
this support. Frankly, there is none. Patches welcome.

.github/workflows/c-cpp.yml

@@ -16,7 +16,7 @@ jobs:
         run: |
           sudo apt-get update
           sudo apt-get upgrade
-          sudo apt-get install python3 python3-pip libgtk-3-dev gcc libgeoip-dev gettext libappstream-dev appstream-util libcurl4-openssl-dev libjson-glib-dev libmrss0-dev libproxy-dev
+          sudo apt-get install python3 python3-pip libgtk-3-dev gcc libgeoip-dev gettext libappstream-dev appstream-util libcurl4-openssl-dev libjson-glib-dev libproxy-dev
           # Meson is too old on ubuntu
           pip3 install --no-input meson ninja
       - name: meson configure