Retire `storage_items!`

The `storage_items!` macro should be removed in favor of `decl_storage_items!`.

[Here](https://github.com/paritytech/substrate/blob/8ef14908ad9d46840db58808a72516ba9fb6602f/srml/support/src/storage/generator.rs#L197) the macro can be found.

While that is done, `StorageList` trait should be removed as well.