remove /cluster/kubeadm.sh and /test/cmd/kubeadm.sh

/cluster/kubeadm.sh is used to find the kubeadm binary.
This file is legacy and is removed.

Remove /test/cmd/kubeadm.sh. This file contains a function that is used
to build kubeadm and invoke "make test". Move the function contents
to hack/make-rules/test-cmd.cmd.

Stop sourcing /test/cmd/kubeadm.sh in /test/cmd/legacy-script.sh.

Also remove the --kubeadm-path invocation as this can be handled
with an env. variable directly.

cluster/kubeadm.sh

@@ -1,42 +0,0 @@
-#!/usr/bin/env bash
-
-# Copyright 2016 The Kubernetes Authors.
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-set -o errexit
-set -o nounset
-set -o pipefail
-
-KUBE_ROOT=${KUBE_ROOT:-$(dirname "${BASH_SOURCE[0]}")/..}
-source "${KUBE_ROOT}/cluster/clientbin.sh"
-
-# If KUBEADM_PATH isn't set, gather up the list of likely places and use ls
-# to find the latest one.
-if [[ -z "${KUBEADM_PATH:-}" ]]; then
-  kubeadm=$( get_bin "kubeadm" "cmd/kubeadm" )
-
-  if [[ ! -x "$kubeadm" ]]; then
-    print_error "kubeadm"
-    exit 1
-  fi
-elif [[ ! -x "${KUBEADM_PATH}" ]]; then
-  {
-    echo "KUBEADM_PATH environment variable set to '${KUBEADM_PATH}', but "
-    echo "this doesn't seem to be a valid executable."
-  } >&2
-  exit 1
-fi
-kubeadm="${KUBEADM_PATH:-${kubeadm}}"
-
-"${kubeadm}" "${@+$@}"