RFR: 2420: Remove closed pr from all entries in targetRefPRMap

[Zhao Song]

In [SKARA-1937](https://bugs.openjdk.org/browse/SKARA-1937), we introduced a feature that pull request bot can re-evaluate all PRs when jcheck configuration changes. When implementing it, I let pull request bot maintains a hash map called targetRefPRMap. The key is the name of targetRef and the value is the list of pr which targets to the targetRef. Currently, when a pr is closed, the bot will unlink it with its current targetRef. There was an issue happened recently, there was an entry {pr/96=[pr/97]} in the hash map, and before pr/97 was integrated, the user retargeted the pr from pr/96 to master, so the bot fails to remove the association between pr/96 and pr/97.

I think the right behavior here is that when pull request bot finds an updated pr, it should unlink this pr with all targetRefs and if this pr is still open, the bot should link the pr with current targetRef.

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Commit messages:
 - update
 - update
 - update
 - SKARA-2420

Changes: https://git.openjdk.org/skara/pull/1701/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1701&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2420
  Stats: 41 lines in 5 files changed: 22 ins; 5 del; 14 mod
  Patch: https://git.openjdk.org/skara/pull/1701.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1701/head:pull/1701

PR: https://git.openjdk.org/skara/pull/1701