Move the temporary private freelist closer to where it's used

src/sweeper-thread.h

Index: src/sweeper-thread.h
diff --git a/src/sweeper-thread.h b/src/sweeper-thread.h
index 96255a0972af61b37543dc294018e01ff2d30073..5013affd84dce31d5345c2e4473737debfa27cf3 100644
--- a/src/sweeper-thread.h
+++ b/src/sweeper-thread.h
@@ -62,8 +62,6 @@ class SweeperThread : public Thread {
   Semaphore stop_semaphore_;
   FreeList free_list_old_data_space_;
   FreeList free_list_old_pointer_space_;
-  FreeList private_free_list_old_data_space_;
-  FreeList private_free_list_old_pointer_space_;
   volatile AtomicWord stop_thread_;
 };