runtime: remove syscall.NewCallbackCDecl on Windows
runtime: remove syscall.NewCallbackCDecl on Windows

It is not possible to use (there is no declaration in package syscall),
and no one seems to care.

Alex Brainman may bring this back properly for Go 1.3.

Fixes issue 6338.

R=golang-dev, r, alex.brainman
CC=golang-dev
https://codereview.appspot.com/14287043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang