RFR: 2562: Disable backport command in open pull requests until Github allows the bot to create labels

[Zhao Song]

In [SKARA-1797](https://bugs.openjdk.org/browse/SKARA-1797), the bot was enhanced to support backport pull request command, but with the implementation, the bot needs to add a customized label to the pull request. Recently, Github did some changes and it no longer allows the bot to create new labels. I need to disable this feature until Github allows the bot to create new labels again.

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Commit messages:
 - SKARA-2562

Changes: https://git.openjdk.org/skara/pull/1731/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1731&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2562
  Stats: 97 lines in 3 files changed: 5 ins; 0 del; 92 mod
  Patch: https://git.openjdk.org/skara/pull/1731.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1731/head:pull/1731

PR: https://git.openjdk.org/skara/pull/1731