Buck download deps are no longer needed

Gerrit has now moved on and Buck isn't the mainstream development
build tool anymore. Whilst we need to keep on supporting
the build of stable branches, it is not a requirement anymore to
have the full dependencies jars locally.

Change-Id: Ic740b018505e00169b9c2a70436bd54375a4bb90
diff --git a/jenkins-docker/slave-buck/Dockerfile b/jenkins-docker/slave-buck/Dockerfile
index 3be5203..a3f3942 100644
--- a/jenkins-docker/slave-buck/Dockerfile
+++ b/jenkins-docker/slave-buck/Dockerfile
@@ -39,7 +39,6 @@
 USER jenkins
 RUN bash -c '. /usr/bin/set-java.sh 8 && \
     cd /tmp &&  git clone --recursive https://gerrit.googlesource.com/gerrit && cd /tmp/gerrit && \
-    which buck && ./tools/download_all.py  && \
     buck build gerrit api plugins:core || true  && \
     buck test --dry-run --no-results-cache --exclude flaky || true  && \
     rm -Rf /tmp/gerrit /home/jenkins/.gerritcodereview/buck-cache/{cache,locally-built-artifacts}'