runtime: remove out-of-date comment

It used to be the case that repeatedly getting one GC pointer and
enqueuing one GC pointer could cause contention on the work buffers as
each operation passed over the boundary of a work buffer. As of
b6c0934a, we use a two buffer cache that prevents this sort of
contention.

Change-Id: I4f1111623f76df9c5493dd9124dec1e0bfaf53b7
Reviewed-on: https://go-review.googlesource.com/18532
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgcmark.go

@@ -835,12 +835,6 @@ func gcDrain(gcw *gcWork, flags gcDrainFlags) {
 			// work barrier reached or tryGet failed.
 			break
 		}
-		// If the current wbuf is filled by the scan a new wbuf might be
-		// returned that could possibly hold only a single object. This
-		// could result in each iteration draining only a single object
-		// out of the wbuf passed in + a single object placed
-		// into an empty wbuf in scanobject so there could be
-		// a performance hit as we keep fetching fresh wbufs.
 		scanobject(b, gcw)
 
 		// Flush background scan work credit to the global