Move away from grpc-ping test image as symlink (#1521)

test/test_images/grpc-ping

@@ -1 +0,0 @@
-./../../vendor/knative.dev/serving/test/test_images/grpc-ping

test/test_images/grpc-ping/release.yaml

@@ -0,0 +1,16 @@
+# Copyright 2021 The Knative 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
+#
+#     https://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.
+
+- image: ko://knative.dev/serving/test/test_images/grpc-ping
+