Bug 1298947 - Remove curl and jq from decision image; r=dustin

curl and jq were previously used to fetch and parse the TC secret.
We now use Python for that. So remove the unused packages.

This reduces the Docker image size by ~10MB.

MozReview-Commit-ID: Nl7fC1aG7w

--HG--
extra : rebase_source : e1bfe76f0e2efd06c2e2e8a94f6c3fb3dc4c0d50

testing/docker/decision/system-setup.sh

@@ -7,8 +7,6 @@ test `whoami` == 'root'
 apt-get update
 apt-get install -y --force-yes --no-install-recommends \
     ca-certificates \
-    curl \
-    jq \
     python \
     sudo