VideoBackendBase: Remove redundant unique_ptr reset

Remove redundant reset of g_presenter in ShutdownShared, which is
already reset earlier in the function.

Source/Core/VideoCommon/VideoBackendBase.cpp

@@ -369,7 +369,6 @@ void VideoBackendBase::ShutdownShared()
   g_vertex_manager.reset();
   g_efb_interface.reset();
   g_widescreen.reset();
-  g_presenter.reset();
   g_gfx.reset();
 
   m_initialized = false;