runtime: remove out-of-date comment

An out-of-date comment snuck in to cc8f5441. Remove it.

Change-Id: I5bc7c17e737d1cabe57b88de06d7579c60ca28ff
Reviewed-on: https://go-review.googlesource.com/12328Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/runtime/mgcsweep.go

@@ -170,7 +170,7 @@ func mSpan_Sweep(s *mspan, preserve bool) bool {
 	cl := s.sizeclass
 	size := s.elemsize
 	res := false
-	nfree := 0 // Set to -1 for large span
+	nfree := 0
 
 	var head, end gclinkptr