Remove parallel sweeping mode, it's no longer used

src/sweeper-thread.cc

Index: src/sweeper-thread.cc
diff --git a/src/sweeper-thread.cc b/src/sweeper-thread.cc
index fee3e92ff43974de780b6686784dbe9966d7fd50..f1641639dbe641e808926f4de0fee0f99c2393a2 100644
--- a/src/sweeper-thread.cc
+++ b/src/sweeper-thread.cc
@@ -76,11 +76,9 @@ bool SweeperThread::SweepingCompleted() {
 
 
 int SweeperThread::NumberOfThreads(int max_available) {
-  if (!FLAG_concurrent_sweeping && !FLAG_parallel_sweeping) return 0;
+  if (!FLAG_concurrent_sweeping) return 0;
   if (FLAG_sweeper_threads > 0) return FLAG_sweeper_threads;
-  if (FLAG_concurrent_sweeping) return max_available - 1;
-  ASSERT(FLAG_parallel_sweeping);
-  return max_available;
+  return max_available - 1;
 }
 
 } }  // namespace v8::internal