openshift/os: drop COSA_SKIP_OVERLAY=1 workaround

This is no longer required after:

coreos/coreos-assembler#3936

Author: jlebon

ci-operator/config/openshift/os/openshift-os-master.yaml

@@ -11,8 +11,6 @@ images:
     # Tell the build scripts that we have setup a directory for unprivileged build
     ENV COSA_DIR=/tmp/cosa
     RUN mkdir -p "${COSA_DIR}"
-    # Prow doesn't support emptydir for jobs today
-    ENV COSA_SKIP_OVERLAY=1
     # Copy the source code
     COPY . .
     # We need to make sure that root can read / write to the COSA_DIR so that