_issue_to_pull_request(body) got the pr_body for us (with logging for
failures, then we did it again in the calling context. We should only
need to fetch this data once and doing it with logging is preferable.
Remove the followup code that is redundant and rely on
_issue_to_pull_request(body) instead.
Change-Id: Iaa1eb4c4d992249504be02efd50c1192fcf99e49