windows: remove unreachable return statement in Proc.Call

Change-Id: I5bb10469bab144f5e42c7bcf6ee8717d3fd0d4e4
Reviewed-on: https://go-review.googlesource.com/47337
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

windows/dll_windows.go

@@ -160,7 +160,6 @@ func (p *Proc) Call(a ...uintptr) (r1, r2 uintptr, lastErr error) {
 	default:
 		panic("Call " + p.Name + " with too many arguments " + itoa(len(a)) + ".")
 	}
-	return
 }
 
 // A LazyDLL implements access to a single DLL.