fix: forget unbanned rejoin

[Akinator31]

This pull request introduces functionality to remove a user from the banned users database table when they rejoin a guild. The main changes add a new database operation and integrate it into the guild member event handler.

Database operations:

• Added a new async function delete_banned_user to banned_users.rs that deletes a banned user record from the banned_users table given a guild_id and user_id.

Event handling:

• Updated the GuildBanHandler to call delete_banned_user when a previously banned user rejoins a guild, and log an error if the deletion fails.