RFR: 2061: Checking for .jcheck/conf fails when pr/X branches are removed

[Zhao Song]

As Erik described in the issue, "The targetRefPRMap introduced in [SKARA-1937](https://bugs.openjdk.org/browse/SKARA-1937) isn't working well with the dependent PR feature. When a pr/X branch is removed, the map may still contains that branch which will cause getPeriodicItems to fail with exception when trying to get .jcheck/conf from the non existing branch.". 

To solve this issue, I think that every pr would only have one targetRef, so before the bot is trying to create an entry for the pr, the bot should delete any existing entry associated with that pr.

Since the operations are all about HashMap, I believe they wouldn't significantly impact the bot's performance.

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Commit messages:
 - SKARA-2061

Changes: https://git.openjdk.org/skara/pull/1568/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1568&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2061
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/skara/pull/1568.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1568/head:pull/1568

PR: https://git.openjdk.org/skara/pull/1568