Revert "Added MG_SHUTDOWN event"

This reverts commit 15a0d819.
There is no point in MG_SHUTDOWN. Thread that calls mg_stop() can
do the same thing.

mongoose.c

@@ -4303,7 +4303,6 @@ static void free_context(struct mg_context *ctx) {
 }
 
 void mg_stop(struct mg_context *ctx) {
-  call_user(fc(ctx), MG_SHUTDOWN);
   ctx->stop_flag = 1;
 
   // Wait until mg_fini() stops

mongoose.h

@@ -59,8 +59,7 @@ enum mg_event {
   MG_EVENT_LOG,     // Mongoose logs an event, request_info.log_message
   MG_INIT_SSL,      // Mongoose initializes SSL. Instead of mg_connection *,
                     // SSL context is passed to the callback function.
-  MG_REQUEST_COMPLETE,  // Mongoose has finished handling the request
-  MG_SHUTDOWN       // Called when Mongoose stops, request_info is empty.
+  MG_REQUEST_COMPLETE  // Mongoose has finished handling the request
 };
 
 // Prototype for the user-defined function. Mongoose calls this function