fix gcloud builds

Signed-off-by: James Strong <strong.james.e@gmail.com>

images/echo/cloudbuild.yaml

@@ -6,8 +6,6 @@ steps:
     entrypoint: bash
     env:
       - DOCKER_CLI_EXPERIMENTAL=enabled
-      - SHORT_SHA=$SHORT_SHA
-      - BASE_REF=$_PULL_BASE_REF
       - REGISTRY=gcr.io/k8s-staging-ingress-nginx
       # default cloudbuild has HOME=/builder/home and docker buildx is in /root/.docker/cli-plugins/docker-buildx
       # set the home to /root explicitly to if using docker buildx
@@ -17,6 +15,3 @@ steps:
     - |
       gcloud auth configure-docker \
       && cd images/echo && make push
-substitutions:
-  _GIT_TAG: "12345"
-  _PULL_BASE_REF: "master"