Remove the source related environment variables

The previous approach tried to clear environment variables so that upstream CI builds did not inherit them. However, doozer fought with that method. There is a new image metadata value that prevents all environment variable injections: https://github.com/openshift/ocp-build-data/blob/9998b9735d6c7364865989d9029976b22223a5e0/images/openshift-golang-builder.yml#L2

images/openshift-golang-builder.Dockerfile

@@ -6,22 +6,6 @@ ENV SUMMARY="RHEL8 based Go builder image for OpenShift ART" \
     GOPATH=/go \
     VERSION="1.14"
 
-ENV SOURCE_GIT_COMMIT= \
-    OS_GIT_COMMIT= \
-    SOURCE_GIT_URL= \
-    SOURCE_GIT_TAG= \
-    OS_GIT_VERSION= \
-    SOURCE_DATE_EPOCH= \
-    BUILD_VERSION= \
-    BUILD_RELEASE= \
-    SOURCE_GIT_TAG= \
-    SOURCE_GIT_URL= \
-    OS_GIT_MAJOR= \
-    OS_GIT_MINOR= \
-    OS_GIT_PATCH= \
-    OS_GIT_TREE_STATE= \
-    SOURCE_GIT_TREE_STATE= 
-
 LABEL summary="$SUMMARY" \
       description="$SUMMARY" \
       io.k8s.description="$SUMMARY" \