Fix builds on ARM arch but keeping ARM cross builds. Fix #638.

plugins/rss/Dockerfile.armhf

@@ -1,8 +1,6 @@
 FROM resin/rpi-raspbian:stretch
 MAINTAINER Kristian Haugene
 
-RUN [ "cross-build-start" ]
-
 # Update packages and install software
 RUN apt-get update \
     && apt-get -y upgrade \
@@ -16,5 +14,3 @@ ENV TRANSMISSION_DOWNLOAD_DIR=/data/completed \
     RSS_URL=**None**
 
 CMD ["/etc/transmission-rss/start.sh"]
-
-RUN [ "cross-build-end" ]