gc.c (clear_all_old): fix a generational GC bug

This issue was caused by 375d22b that introduced a serious mark
leakage in the generational GC mode.

Author: matz

src/gc.c

@@ -1240,14 +1240,12 @@ clear_all_old(mrb_state *mrb, mrb_gc *gc)
     /* finish the half baked GC */
     incremental_gc_finish(mrb, gc);
   }
-  else {
-    /* Sweep the dead objects, then reset all the live objects
-     * (including all the old objects, of course) to white. */
-    gc->generational = FALSE;
-    prepare_incremental_sweep(mrb, gc);
-    incremental_gc_finish(mrb, gc);
-    gc->generational = TRUE;
-  }
+  /* Sweep the dead objects, then reset all the live objects
+   * (including all the old objects, of course) to white. */
+  gc->generational = FALSE;
+  prepare_incremental_sweep(mrb, gc);
+  incremental_gc_finish(mrb, gc);
+  gc->generational = TRUE;
   /* The gray objects have already been painted as white */
   gc->atomic_gray_list = gc->gray_list = NULL;
 }