go.tools/go/ssa: remove windows test exclusion

Issue 7189 was likely fixed with revision ed6ecfe1bc4e .

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/59230044

go/ssa/stdlib_test.go

@@ -49,11 +49,6 @@ func allPackages() []string {
 }
 
 func TestStdlib(t *testing.T) {
-	// TODO(brainman): delete when issue 7189 is fixed.
-	if runtime.GOOS == "windows" {
-		t.Skip("skipping broken test on windows; see golang.org/issue/7189")
-	}
-
 	// Load, parse and type-check the program.
 	t0 := time.Now()