[buildbot] Remove bot message about linking PRs

Summary:
Since summary edits already appear as <user> edited the summary of this revision. (Show Details),
this message does not add much additional value. It's informative for new devs, but noise for just about
everyone else.

Test Plan: pytest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7813