Output shouldn't have gRPC Authors copyright header

[kriswuollett]

Other gPRC generation plugins like the Official C++ one do not add a copyright header to the generated files that say it is copyright by the "gRPC Authors" like this Swift program does:

grpc-swift/Sources/protoc-gen-grpc-swift/Generator.swift

Lines 110 to 135 in 76d4ec1

	private func printMain() {
	self.printer.print("""
	//
	// DO NOT EDIT.
	// swift-format-ignore-file
	//
	// Generated by the protocol buffer compiler.
	// Source: \(self.file.name)
	//
	//
	// Copyright 2018, gRPC Authors All rights reserved.
	//
	// 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.
	//\n
	""")

Although the code requires linking against the Apache-licensed gRPC and Protobuf libraries, as in commonly-used practice of code generators, the output should be owned by the the owner of the input files and the executor of the code generator. Instead it should be up to the author of the .proto file to provide their own header through #1610.