Separate app-manager and app-framework from WAMR (#3129)

As planned, the app-manager and app-framework are to be migrated to
https://github.com/bytecodealliance/wamr-app-framework.

ps.
https://github.com/bytecodealliance/wasm-micro-runtime/issues/2329
https://github.com/bytecodealliance/wasm-micro-runtime/wiki/TSC-meeting-notes

core/app-mgr/app-manager/platform/darwin/app_mgr_darwin.c

@@ -1 +0,0 @@
-#include "../linux/app_mgr_linux.c"

core/app-mgr/app-manager/platform/linux/app_mgr_linux.c

@@ -1,46 +0,0 @@
-/*
- * Copyright (C) 2019 Intel Corporation.  All rights reserved.
- * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
- */
-
-#include "app_manager.h"
-
-void *
-app_manager_timer_create(void (*timer_callback)(void *),
-                         watchdog_timer *wd_timer)
-{
-    /* TODO */
-    return NULL;
-}
-
-void
-app_manager_timer_destroy(void *timer)
-{
-    /* TODO */
-}
-
-void
-app_manager_timer_start(void *timer, int timeout)
-{
-    /* TODO */
-}
-
-void
-app_manager_timer_stop(void *timer)
-{
-    /* TODO */
-}
-
-watchdog_timer *
-app_manager_get_wd_timer_from_timer_handle(void *timer)
-{
-    /* TODO */
-    return NULL;
-}
-
-int
-app_manager_signature_verify(const uint8_t *file, unsigned int file_len,
-                             const uint8_t *signature, unsigned int sig_size)
-{
-    return 1;
-}