Deserialize the correct data from the GUILD_DELETE event

[arqunis]

Fixes #1023.

As explained in my comment, we expect to receive data from Discord that is a PartialGuild, while in truth, Discord sends us GuildUnavailable on the GUILD_DELETE event.

[lapin-b]

I have just tested the fix, works properly on my side; the event handler is called, call that is shown in the last line of this log excerpt:

INFO [serenity::gateway::shard] handle_event; event=Ok(Dispatch(20, GuildDelete(GuildDeleteEvent { guild: GuildUnavailable { id: GuildId(572913620132954122), unavailable: false }, _nonexhaustive: () })))
INFO [serenity::gateway::shard] handle_gateway_dispatch; seq=20 event=GuildDelete(GuildDeleteEvent { guild: GuildUnavailable { id: GuildId(572913620132954122), unavailable: false }, _nonexhaustive: () })
INFO [serenity::client::dispatch] handle_event; runner_tx=UnboundedSender(Some(UnboundedSenderInner { inner: UnboundedInner { state: 9223372036854775808, message_queue: Queue { head: 0x55a2e3ffc2f0, tail: UnsafeCell }, num_senders: 2, recv_task: AtomicWaker } })) shard_id=0
INFO [discord_bot_rs::event_handler] Deleting configuration for guild 572913620132954122