Remove travis support and unused buildmode

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

hack/util

@@ -8,28 +8,19 @@
 : ${CACHEDIR_FROM=}
 : ${CACHEDIR_TO=}
 
-newerEqualThan() { # $1=minimum wanted version $2=actual-version
-  [ "$1" = "$(printf "$1\n$2" | sort -V | head -n 1)" ]
-}
+if [ "$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
 
-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
-
 buildxCmd() {
   if docker buildx version >/dev/null 2>&1; then
     set -x