Remove unused lambda capture in chain_api_plugin

warning b-gone

plugins/chain_api_plugin/chain_api_plugin.cpp

@@ -50,7 +50,7 @@ struct async_result_visitor : public fc::visitor<std::string> {
 
 #define CALL_ASYNC(api_name, api_handle, api_namespace, call_name, call_result, http_response_code) \
 {std::string("/v1/" #api_name "/" #call_name), \
-   [this, api_handle](string, string body, url_response_callback cb) mutable { \
+   [api_handle](string, string body, url_response_callback cb) mutable { \
       if (body.empty()) body = "{}"; \
       api_handle.validate(); \
       api_handle.call_name(fc::json::from_string(body).as<api_namespace::call_name ## _params>(),\