RFR: Move hgupdate-sync label functionality to SyncLabelBot

[Robin Westberg]

Move the responsibility of updating the hgupdate-sync label to the SyncLabelBot.

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Commit messages:
 - Move hgupdate-sync label functionality to SyncLabelBot

Changes: https://git.openjdk.java.net/skara/pull/1021/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1021&range=00
  Stats: 379 lines in 10 files changed: 229 ins; 136 del; 14 mod
  Patch: https://git.openjdk.java.net/skara/pull/1021.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1021/head:pull/1021

PR: https://git.openjdk.java.net/skara/pull/1021