Remove a TODO about supporting stdin.

Passing "/dev/stdin" works just fine.
diff --git a/golint/golint.go b/golint/golint.go
index 2c9d7ce..d634fae 100644
--- a/golint/golint.go
+++ b/golint/golint.go
@@ -24,7 +24,6 @@
 func main() {
 	flag.Parse()
 
-	// TODO(dsymonds): Support linting of stdin.
 	for _, filename := range flag.Args() {
 		if isDir(filename) {
 			lintDir(filename)