runtime: remove unnecessary traceNextGC

Commit d7e0ad4b removed the next_gc manipulation from mSpan_Sweep, but
left in the traceNextGC() for recording the updated next_gc
value. Remove this now unnecessary call.

Change-Id: I28e0de071661199be9810d7bdcc81ce50b5a58ae
Reviewed-on: https://go-review.googlesource.com/8894Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgcsweep.go

@@ -293,7 +293,6 @@ func mSpan_Sweep(s *mspan, preserve bool) bool {
 	}
 	if trace.enabled {
 		traceGCSweepDone()
-		traceNextGC()
 	}
 	return res
 }