Remove disabling of warning 4660 on MSVC

This warning no longer exists since at least MSVC.NET 2003. It's
reported to have existed on MSVC 6 and older only.

Task-number: QTBUG-35815
Change-Id: Ifb35d027edc3c1cbadac9a0d0a9c26524c52c87f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>