RFR: 2201: Only fetch comments once in PullRequestWorkItem in pr bot

[Zhao Song]

In pr bot, we are still fetching comments from the remote forge instead of using the cache after [SKARA-1709](https://bugs.openjdk.org/browse/SKARA-1709).

-------------

Commit messages:
 - SKARA-2201

Changes: https://git.openjdk.org/skara/pull/1622/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1622&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2201
  Stats: 8 lines in 2 files changed: 0 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/skara/pull/1622.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1622/head:pull/1622

PR: https://git.openjdk.org/skara/pull/1622