fix(ReactionCollector): check for channel.threads

[kyranet]

Please describe the changes this PR makes and why it should be merged:

Message#channel becomes null when the channel is deleted, so we cannot rely on it for handling channel deletions of a thread's parent. However, we can do the reverse and check whether the message's channel ID is a thread contained by the deleted channel.

Fixes #7066

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

[vladfrangu]

As long as it works with threads being deleted as well as normal channels and messages from a thread in a channel, and you make sure this isn't an issue in other collectors, LGTM