refactor(app): remove handlers.h

handlers.h isn't referenced elsewhere.

PR: #526

app/include/zmk/handlers.h

@@ -1,12 +0,0 @@
-/*
- * Copyright (c) 2020 The ZMK Contributors
- *
- * SPDX-License-Identifier: MIT
- */
-
-#pragma once
-
-#include <zmk/keymap.h>
-#include <zmk/keys.h>
-
-void zmk_handle_key(struct zmk_key_event key_event);