Unify testutil.Unmount on Windows and Unix

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

pkg/testutil/helpers_windows.go

@@ -25,8 +25,3 @@ func RequiresRoot(t testing.TB) {
 // RequiresRootM is similar to RequiresRoot but intended to be called from *testing.M.
 func RequiresRootM() {
 }
-
-// Unmount unmounts a given mountPoint and sets t.Error if it fails
-// Does nothing on Windows
-func Unmount(t *testing.T, mountPoint string) {
-}