Avoid possible memory leak in the mInterMessageBoxe field

apps/openmw/mwgui/messagebox.cpp

@@ -28,10 +28,7 @@ namespace MWGui
 
     MessageBoxManager::~MessageBoxManager ()
     {
-        for (MessageBox* messageBox : mMessageBoxes)
+        MessageBoxManager::clear();
-        {
-            delete messageBox;
-        }
     }
 
     int MessageBoxManager::getMessagesCount()