hack/util: remove unused buildmode

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

.travis.yml

@@ -6,7 +6,6 @@ env:
     - DOCKER_CHANNEL="stable"
     - DOCKER_CLI_EXPERIMENTAL="enabled"
     - PLATFORMS="linux/amd64,linux/arm/v7,linux/arm64,linux/s390x,linux/ppc64le"
-    - PREFER_BUILDCTL="1"
 
 before_install:
   # update docker

hack/util

@@ -5,37 +5,23 @@ export BUILDX_NO_DEFAULT_LOAD=true
 : ${PREFER_LEGACY=}
 : ${CI=}
 : ${GITHUB_ACTIONS=}
-: ${DOCKER_BUILDKIT=}
 : ${TRAVIS_EVENT_TYPE=}
 : ${TRAVIS_BRANCH=}
 : ${CACHE_DIR=}
 
+if [ "$PREFER_BUILDCTL" = "1" ]; then
+  echo >&2 "WARNING: PREFER_BUILDCTL is no longer supported. Ignoring."
+fi
+
+if [ "$PREFER_LEGACY" = "1" ]; then
+  echo >&2 "WARNING: PREFER_LEGACY is no longer supported. Ignoring."
+fi
+
 progressFlag=""
 if [ "$CI" = "true" ]; then
   progressFlag="--progress=plain"
 fi
 
-newerEqualThan() { # $1=minimum wanted version $2=actual-version
-  [ "$1" = "$(printf "$1\n$2" | sort -V | head -n 1)" ]
-}
-
-buildmode="legacy"
-if [ "$PREFER_BUILDCTL" = "1" ]; then
-  buildmode="buildkit"
-else
-  serverVersion=$(docker info --format '{{.ServerVersion}}')
-  experimental=$(docker info --format '{{.ExperimentalBuild}}')
-  if [ "$PREFER_LEGACY" != "1" ] && (newerEqualThan "18.09" $serverVersion ||
-    (newerEqualThan "18.06" $serverVersion && [ "true" = "$experimental" ]) ||
-    [ "$DOCKER_BUILDKIT" = "1" ]); then
-    buildmode="docker-buildkit"
-  fi
-fi
-if [ "$buildmode" = "legacy" ]; then
-  echo >&2 "ERROR: \"legacy\" buildmode is no longer supported. Please enable DOCKER_BUILDKIT or install standalone BuildKit"
-  exit 1
-fi
-
 buildxCmd() {
   if docker buildx version >/dev/null 2>&1; then
     set -x