mime: do not test for .wav on windows

Even builders don't have that mime type

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/7314106

src/pkg/mime/type_windows.go

@@ -58,7 +58,6 @@ func initMime() {
 
 func initMimeForTests() map[string]string {
 	return map[string]string{
-		".wav": "audio/wav",
 		".png": "image/png",
 	}
 }