Remove unused core/app-mgr folder (#3186)

The app-manager and app-framework have been migrated to:
https://github.com/bytecodealliance/wamr-app-framework

core/app-mgr/README.md

@@ -1,8 +0,0 @@
-# Remote application management
-
-The WAMR application manager supports [remote application management](../../core/app-mgr) from the host environment or the cloud through any physical communications such as TCP, UPD, UART, BLE, etc. Its modular design makes it able to support application management for different managed runtimes.
-
-The tool [host_tool](../../test-tools/host-tool) communicates to the WAMR app manager for installing/uninstalling the WASM applications on companion chip from the host system. And the [IoT App Store Demo](../../test-tools/IoT-APP-Store-Demo/) shows the conception of remotely managing the device applications from the cloud.
-
-
-<img src="../../doc/pics/wamr-arch.JPG" width="80%">

core/app-mgr/app-manager/platform/zephyr/app_mgr_zephyr.c

@@ -1,70 +0,0 @@
-/*
- * Copyright (C) 2019 Intel Corporation.  All rights reserved.
- * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
- */
-
-#include "app_manager.h"
-#include "bh_platform.h"
-#include <autoconf.h>
-
-#if KERNEL_VERSION_NUMBER < 0x030200 /* version 3.2.0 */
-#include <zephyr.h>
-#include <kernel.h>
-#else
-#include <zephyr/kernel.h>
-#endif
-
-#if 0
-#include <sigverify.h>
-#endif
-typedef struct k_timer_watchdog {
-    struct k_timer timer;
-    watchdog_timer *wd_timer;
-} k_timer_watchdog;
-
-void *
-app_manager_timer_create(void (*timer_callback)(void *),
-                         watchdog_timer *wd_timer)
-{
-    struct k_timer_watchdog *timer =
-        APP_MGR_MALLOC(sizeof(struct k_timer_watchdog));
-
-    if (timer) {
-        k_timer_init(&timer->timer, (void (*)(struct k_timer *))timer_callback,
-                     NULL);
-        timer->wd_timer = wd_timer;
-    }
-
-    return timer;
-}
-
-void
-app_manager_timer_destroy(void *timer)
-{
-    APP_MGR_FREE(timer);
-}
-
-void
-app_manager_timer_start(void *timer, int timeout)
-{
-    k_timer_start(timer, Z_TIMEOUT_MS(timeout), Z_TIMEOUT_MS(0));
-}
-
-void
-app_manager_timer_stop(void *timer)
-{
-    k_timer_stop(timer);
-}
-
-watchdog_timer *
-app_manager_get_wd_timer_from_timer_handle(void *timer)
-{
-    return ((k_timer_watchdog *)timer)->wd_timer;
-}
-#if 0
-int app_manager_signature_verify(const uint8_t *file, unsigned int file_len,
-        const uint8_t *signature, unsigned int sig_size)
-{
-    return signature_verify(file, file_len, signature, sig_size);
-}
-#endif