misc/cgo/testcshared: delete temporary directory in test

The deferred os.RemoveAll was accidentally committed as commented out
in the original https://golang.org/cl/87158.

Updates #30500

Change-Id: Idc5195816d7978253760dbfd78fde6d22c456296
Reviewed-on: https://go-review.googlesource.com/c/164858
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

misc/cgo/testcshared/cshared_test.go

@@ -521,7 +521,7 @@ func TestCachedInstall(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	// defer os.RemoveAll(tmpdir)
+	defer os.RemoveAll(tmpdir)
 
 	copyFile(t, filepath.Join(tmpdir, "src", "testcshared", "go.mod"), "go.mod")
 	copyFile(t, filepath.Join(tmpdir, "src", "testcshared", "libgo", "libgo.go"), filepath.Join("libgo", "libgo.go"))