relay: remove a few outdated comments

There were 3 comments which mentioned relay stream restart. It
could happen when replicas ignored data from non-leaders. A newly
elected leader had to re-send data, which others ignored, while
the source wasn't a leader.

Was removed in the commit 6336ea26
("refactoring: remove relay_restart_recovery as unneeded").

This is fixed now, because otherwise some of these comments would
need to be updated by a next commit in this series.

In scope of #7253

NO_DOC=refactoring
NO_CHANGELOG=refactoring
NO_TEST=refactoring