runtime: remove stale comment about getcallerpc/sp

Getcallerpc/sp no longer takes argument.

Change-Id: I80b30020e798990c59c8ffd0a4e078af6a75aea0
Reviewed-on: https://go-review.googlesource.com/109696Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/stubs.go

@@ -213,10 +213,7 @@ func publicationBarrier()
 // the call to f (where f will return).
 //
 // The call to getcallerpc and getcallersp must be done in the
-// frame being asked about. It would not be correct for f to pass &arg1
-// to another function g and let g call getcallerpc/getcallersp.
-// The call inside g might return information about g's caller or
-// information about f's caller or complete garbage.
+// frame being asked about.
 //
 // The result of getcallersp is correct at the time of the return,
 // but it may be invalidated by any subsequent call to a function