Revert "ns_subclasses refcount accesses need to be atomic (#15083)" (#15138)

This reverts commit 2998c8d6b9.

We need to find a better way to fix this bug. Even with this refcount
change, errors were still being seen in CI. For now we need to remove
this failing test.

class.c

@@ -671,7 +671,7 @@ rb_class_classext_free_subclasses(rb_classext_t *ext, VALUE klass, bool replacin
     }
     VM_ASSERT(
         rb_box_subclasses_ref_count(anchor->box_subclasses) > 0,
-        "box_subclasses refcount (%p) %d", anchor->box_subclasses, rb_box_subclasses_ref_count(anchor->box_subclasses));
+        "box_subclasses refcount (%lp) %d", anchor->box_subclasses, rb_box_subclasses_ref_count(anchor->box_subclasses));
     st_delete(tbl, &box_id, NULL);
     rb_box_subclasses_ref_dec(anchor->box_subclasses);
     xfree(anchor);

internal/class.h

@@ -28,29 +28,29 @@
 #endif
 
 struct rb_box_subclasses {
-    rb_atomic_t refcount;
+    long refcount;
     struct st_table *tbl;
 };
 typedef struct rb_box_subclasses rb_box_subclasses_t;
 
-static inline rb_atomic_t
+static inline long
 rb_box_subclasses_ref_count(rb_box_subclasses_t *box_sub)
 {
-    return ATOMIC_LOAD_RELAXED(box_sub->refcount);
+    return box_sub->refcount;
 }
 
 static inline rb_box_subclasses_t *
 rb_box_subclasses_ref_inc(rb_box_subclasses_t *box_sub)
 {
-    RUBY_ATOMIC_FETCH_ADD(box_sub->refcount, 1);
+    box_sub->refcount++;
     return box_sub;
 }
 
 static inline void
 rb_box_subclasses_ref_dec(rb_box_subclasses_t *box_sub)
 {
-    rb_atomic_t was = RUBY_ATOMIC_FETCH_SUB(box_sub->refcount, 1);
-    if (was == 1) {
+    box_sub->refcount--;
+    if (box_sub->refcount == 0) {
         st_free_table(box_sub->tbl);
         xfree(box_sub);
     }

test/ruby/test_class.rb

@@ -887,19 +887,4 @@ CODE
       class C; end
     end;
   end
-
-  def test_subclasses_refcount_in_ractors
-    assert_ractor "#{<<~"begin;"}\n#{<<~'end;'}"
-    begin;
-      rs = []
-      8.times do
-        rs << Ractor.new do
-          5_000.times do
-            Class.new
-          end
-        end
-      end
-      rs.each(&:join)
-    end;
-  end
 end