Remove /twitchemotes/set/:setID endpoint

[jupjohn]

Chatterino no longer makes use of the /twitchemotes/set/:setID endpoint provided by this API. Could this be removed?

@pajlada you might have metrics on external callers. I doubt anyone else is using it but would suck to drop it with 0 notice

[pajlada]

I don't have metrics, once we have metrics I would be more comfortable deprecating endpoints

[M4tthewDE]

As it has been over 2 months since adding additional metrics, it might be a good time revisiting this issue.

[pajlada]

I can confirm that there have been no requests to the /twitchemotes/set/:setID endpoint since we started tracking this, other than the requests I made just now to make sure it was being tracked