mime: remove ".wav" from tests (to fix build)

It looks like not all computers have
mime type for .wav extension.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5415045

src/pkg/mime/type_windows.go

@@ -57,6 +57,5 @@ func initMimeForTests() map[string]string {
 	return map[string]string{
 		".bmp": "image/bmp",
 		".png": "image/png",
-		".wav": "audio/wav",
 	}
 }