On delete, always set promotions == -2 and node == nil

Also, item.promotions doesn't need to be loaded/stored using atomic. Once upon a
time it did. Cache was updated long ago to not use atomic operations on it, but
LayeredCache wasn't. They are both consistent now (they don't use atomic
operations).

Fixes: https://github.com/karlseguin/ccache/issues/76

cache.go

@@ -394,6 +394,8 @@ func (c *Cache[T]) doDelete(item *Item[T]) {
 			c.onDelete(item)
 		}
 		c.list.Remove(item.node)
+		item.node = nil
+		item.promotions = -2
 	}
 }