VertexManagerBase: Get rid of static behavior

Source/Core/VideoCommon/Fifo.cpp

@@ -390,7 +390,7 @@ void RunGpuLoop()
 
           // The fifo is empty and it's unlikely we will get any more work in the near future.
           // Make sure VertexManager finishes drawing any primitives it has stored in it's buffer.
-          VertexManagerBase::Flush();
+          g_vertex_manager->Flush();
         }
       },
       100);