hack/util: disallow using DOCKER_BUILDKIT instead of buildx

DOCKER_BUILDKIT CLI is slightly different from buildx.
It is hard to maintain DOCKER_BUILDKIT as a buildx alternative.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

hack/util

@@ -28,11 +28,8 @@ buildxCmd() {
   elif buildx version >/dev/null 2>&1; then
     set -x
     buildx "$@" $progressFlag
-  elif docker version >/dev/null 2>&1; then
-    set -x
-    DOCKER_BUILDKIT=1 docker "$@" $progressFlag
   else
-    echo >&2 "ERROR: Please enable DOCKER_BUILDKIT or install standalone buildx"
+    echo >&2 "ERROR: Please install buildx"
     exit 1
   fi
 }