Remove legacy restart code

runtime/bin/error_exit.h

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef RUNTIME_BIN_ERROR_EXIT_H_
 #define RUNTIME_BIN_ERROR_EXIT_H_
 
 namespace dart {
 namespace bin {
 
 // Exit code indicating an internal Dart Frontend error.
 static const int kDartFrontendErrorExitCode = 252;
 // Exit code indicating an API error.
 static const int kApiErrorExitCode = 253;
 // Exit code indicating a compilation error.
 static const int kCompilationErrorExitCode = 254;
 // Exit code indicating an unhandled error that is not a compilation error.
 static const int kErrorExitCode = 255;
-// Exit code indicating a vm restart request.  Never returned to the user.
-static const int kRestartRequestExitCode = 1000;
 
 void ErrorExit(int exit_code, const char* format, ...);
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // RUNTIME_BIN_ERROR_EXIT_H_