Remove the msvc_on_amd64 function

Which `distutils/msvccompiler.py` bug is fixed by the `msvc_on_amd64` function? Numpy and other extensions build OK for me without this function on latest Python 2.6, 2.7, 3.2, to 3.3 using Visual Studio 2008 and 2010. The `Forcing DISTUTILS_USE_SDK=1` printed when importing numpy.distutils is annoying. The use of `DISTUTILS_USE_SDK` to build with the Platform SDK is documented at http://docs.python.org/2/distutils/apiref.html#module-distutils.msvccompiler. Why change the documented behavior?