feat(openthread): reorganize source file structure

components/openthread/src/esp_openthread_platform.cpp

@@ -20,7 +20,6 @@
 #include "esp_partition.h"
 #include "common/code_utils.hpp"
 #include "common/logging.hpp"
-#include "core/instance/instance.hpp"
 #include "freertos/FreeRTOS.h"
 #include "freertos/queue.h"
 #include "openthread/cli.h"
@@ -166,11 +165,6 @@ exit:
     return ret;
 }
 
-otInstance *esp_openthread_get_instance(void)
-{
-    return (otInstance *)&ot::Instance::Get();
-}
-
 esp_err_t esp_openthread_platform_deinit(void)
 {
     ESP_RETURN_ON_FALSE(s_openthread_platform_initialized, ESP_ERR_INVALID_STATE, OT_PLAT_LOG_TAG,