ci: remove building of stabber

It's not run anyways.
See https://github.com/profanity-im/profanity/issues/789

Dockerfile.ubuntu

@@ -33,7 +33,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 RUN mkdir -p /usr/src/{stabber,libstrophe,profanity}
 WORKDIR /usr/src
 
-RUN git clone https://github.com/boothj5/stabber
+#RUN git clone https://github.com/boothj5/stabber
 RUN git clone https://github.com/strophe/libstrophe
 
 # TODO: Re-enable once libmicrohttpd-dev has been updated.