Fixup event loop comments

Summary:
The event loop was decoupled from Avalanche some time ago and these
comments are leftovers from that.

Test Plan:
Read it.
grep -rni ava eventloop.* returns no results

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7555