multiple: remove support for Go 1.11 (#4700)

internal/resolver/dns/go113.go

@@ -1,33 +0,0 @@
-// +build go1.13
-
-/*
- *
- * Copyright 2019 gRPC 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.
- *
- */
-
-package dns
-
-import "net"
-
-func init() {
-	filterError = func(err error) error {
-		if dnsErr, ok := err.(*net.DNSError); ok && dnsErr.IsNotFound {
-			// The name does not exist; not an error.
-			return nil
-		}
-		return err
-	}
-}