Make during_compacting flag in GC one bit

Commit c32218d turned during_compacting
flag to 2 bits to support the case when there is no write barrier. But
commit 32b7dcf changed compaction to
always enable the write barrier. This commit cleans up some of the
leftover code.

gc.c

@@ -715,7 +715,7 @@ typedef struct rb_objspace {
 	unsigned int dont_gc : 1;
 	unsigned int dont_incremental : 1;
 	unsigned int during_gc : 1;
-        unsigned int during_compacting : 2;
+        unsigned int during_compacting : 1;
 	unsigned int gc_stressful: 1;
 	unsigned int has_hook: 1;
 	unsigned int during_minor_gc : 1;
@@ -8915,7 +8915,7 @@ gc_start(rb_objspace_t *objspace, unsigned int reason)
     objspace->flags.immediate_sweep = !!(reason & GPR_FLAG_IMMEDIATE_SWEEP);
 
     /* Explicitly enable compaction (GC.compact) */
-    objspace->flags.during_compacting = (!!(reason & GPR_FLAG_COMPACT) << 1);
+    objspace->flags.during_compacting = !!(reason & GPR_FLAG_COMPACT);
 
     if (!heap_allocated_pages) return FALSE; /* heap is not ready */
     if (!(reason & GPR_FLAG_METHOD) && !ready_to_gc(objspace)) return TRUE; /* GC is not allowed */