ci: remove disabled topic-commenter.yml

Replaced by .github/workflows/explore-triage-commenter.yml in the
preceding commit. The original commenter has been disabled (paths
gated to ENOSUCHPATH) since it failed on fork PRs due to permissions
on the pull_request trigger; the replacement uses pull_request_target
and reads PR head content via the API instead of checking out, which
sidesteps the issue entirely.

Author: kenyonj