runtime: remove out of date comment

Change-Id: I0ad1a81a235c7c067fea2093bbeac4e06a233c10
Reviewed-on: https://go-review.googlesource.com/7661Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgcmark.go

@@ -178,7 +178,6 @@ func gchelpwork() {
 		// hanging off the P where this is being run.
 		// scanstack(gp)
 	case _GCmark:
-		// Get a full work buffer and empty it.
 		// drain your own currentwbuf first in the hopes that it will
 		// be more cache friendly.
 		var gcw gcWork