build: Drop the comment workflow

The workflow introduced in
citynetwork#218, intended to facilitate
pull requests by adding a comment in case the PR produced any build
errors, failed on its first test. A build failure on
citynetwork#217 did cause the action to
fire, but it immediately failed with the following message:

Run quipper/[email protected]
No open pull requests found with packettoobig-patch-1 branch

Since a pull request from that branch obviously did exist, it follows
that the action isn't working as intended, so drop it and rely on
per-user build notifications, set via
https://github.com/settings/notifications, instead.