xmpp: no need to add backports repository

It's now already there since it's in base.

prosody/Dockerfile

@@ -1,7 +1,6 @@
 FROM jitsi/base
 
 RUN \
-	echo "deb http://ftp.debian.org/debian stretch-backports main" > /etc/apt/sources.list.d/backports.list && \
 	apt-dpkg-wrap apt-get update && \
 	apt-dpkg-wrap apt-get install -t stretch-backports -y prosody && \
 	apt-cleanup && \