runtime: remove incorrect TODO added in CL 16035

I've already turned away one attempt to remove this field.
As the comment above the struct says, many tools know the layout.
The field cannot simply be removed.

It was one thing to remove the fields name, but the TODO should
not have been added.

Change-Id: If40eacf0eb35835082055e129e2b88333a0731b9
Reviewed-on: https://go-review.googlesource.com/17741Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/runtime2.go

@@ -497,7 +497,7 @@ type _func struct {
 	nameoff int32   // function name
 
 	args int32 // in/out args size
-	_    int32 // Previously: legacy frame size. TODO: Remove this.
+	_    int32 // previously legacy frame size; kept for layout compatibility
 
 	pcsp      int32
 	pcfile    int32