fix: properly remove deleted channel from cache

[AstreaTSS]

What type of pull request is this?

• Non-breaking code change
• Breaking code change
• Documentation change/addition
• Tests change

Description

The processor for channel deletion was not properly deleting the channel from the cache, instead just deleting from the guild's list of channels. This change makes sure that not even bot.get_channel or the like can get the deleted channel.

Changes

• Use the delete_channel function in the cache rather than trying to handle the deletion logic in the channel deletion processor.
• Update the comment for this to clarify what's going on with the fix.

Checklist

• I've formatted my code with Black
• I've ensured my code works on Python 3.10.x
• I've tested my code