runtime: delete todo of the list field for mspan

Change-Id: I10a3308c19da08d2ff0c8077bb74ad888ee04fea
GitHub-Last-Rev: 3e95b71
GitHub-Pull-Request: golang#64077
Reviewed-on: https://go-review.googlesource.com/c/go/+/541755
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

src/runtime/mheap.go

@@ -399,7 +399,7 @@ type mspan struct {
 	_    sys.NotInHeap
 	next *mspan     // next span in list, or nil if none
 	prev *mspan     // previous span in list, or nil if none
-	list *mSpanList // For debugging. TODO: Remove.
+	list *mSpanList // For debugging.
 
 	startAddr uintptr // address of first byte of span aka s.base()
 	npages    uintptr // number of pages in span