Avoid extra Guild removals in Ready handler (#3331)

This code was removing guilds from cache which the current shard is
responsible for, but wasn't received in the unavailable payload.

This code path could only be hit under extreme circumstances, such as
unsuccessful resumes, and since we cannot rely on cache consistency
after an unsuccessful resume anyway this does more bad than good.

To add to the motivation for doing this, out of the libraries checked
(twilight, discord.py, hikari) none do this either.

Author: GnomedDev

src/cache/event.rs

@@ -1,5 +1,3 @@
-use std::collections::HashSet;
-
 use super::{Cache, CacheUpdate};
 use crate::model::channel::{GuildChannel, Message};
 use crate::model::event::{
@@ -474,27 +472,8 @@ impl CacheUpdate for ReadyEvent {
             cache.unavailable_guilds.insert(unavailable.id, ());
         }
 
-        // We may be removed from some guilds between disconnect and ready, so handle that.
-        let mut guilds_to_remove = vec![];
-        let ready_guilds_hashset =
-            self.ready.guilds.iter().map(|status| status.id).collect::<HashSet<_>>();
         let shard_data = self.ready.shard.unwrap_or_else(|| ShardInfo::new(ShardId(1), 1));
 
-        for guild_entry in cache.guilds.iter() {
-            let guild = guild_entry.key();
-            // Only handle data for our shard.
-            if crate::utils::shard_id(*guild, shard_data.total) == shard_data.id.0
-                && !ready_guilds_hashset.contains(guild)
-            {
-                guilds_to_remove.push(*guild);
-            }
-        }
-        if !guilds_to_remove.is_empty() {
-            for guild in guilds_to_remove {
-                cache.guilds.remove(&guild);
-            }
-        }
-
         {
             let mut cached_shard_data = cache.shard_data.write();
             cached_shard_data.total = shard_data.total;