feature: Remove DM cache and fix references

[SubZero0]

Summary

Since GWv8 doesn't send channel creates for DMs anymore, I decided to stop caching them altogether.
This change should also have a positive impact on the cache size since DMs would never be removed from them while keeping a reference to the user, that would also be kept in memory forever.

Now the DMs and their authors won't be cached because of this change, the method GetOrCreateDMAsync was renamed to CreateDMAsync as it won't ever be cached.

A wrong method call in the client would also cause user objects to never be removed from the cache, this was also fixed.

Changes

• DMs won't be cached anymore, so all methods related to cache in a SocketDMChannel will return null or do a rest request if allowed
• GetOrCreateDMChannelAsync renamed to CreateDMChannelAsync
• Remove wrong method call in the client, causing user objects to never be removed from the cache