[Windows] Remove header guard from generated key map (#140082)

The generated file is a `.cc` file that does not need header guards. This change updates the template that's used to generate that `.cc` file.

No tests as this is a refactoring with no semantic changes.

The generated `.cc` file in the engine will be updated by https://github.com/flutter/engine/pull/48993

dev/tools/gen_keycodes/data/windows_flutter_key_map_cc.tmpl

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef FLUTTER_SHELL_PLATFORM_WINDOWS_FLUTTER_KEY_MAP_H_
-#define FLUTTER_SHELL_PLATFORM_WINDOWS_FLUTTER_KEY_MAP_H_
-
 #include "flutter/shell/platform/windows/keyboard_key_embedder_handler.h"
 
 #include <map>
@@ -40,5 +37,3 @@ std::map<uint64_t, uint64_t> KeyboardKeyEmbedderHandler::scanCodeToLogicalMap_ =
 @@@MASK_CONSTANTS@@@
 
 }  // namespace flutter
-
-#endif