runtime: rename _func.frame to make it clear it's deprecated and unused.

When I saw that it was labelled "legacy", I went looking for users of it
to see how it was still used. But there aren't any. Save the next person
the trouble.

Change-Id: I921dd6c57b60331c9816542272555153ac133c02
Reviewed-on: https://go-review.googlesource.com/16035Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/runtime2.go

@@ -513,8 +513,8 @@ type _func struct {
 	entry   uintptr // start pc
 	nameoff int32   // function name
 
-	args  int32 // in/out args size
-	frame int32 // legacy frame size; use pcsp if possible
+	args int32 // in/out args size
+	_    int32 // Previously: legacy frame size. TODO: Remove this.
 
 	pcsp      int32
 	pcfile    int32