RFR: Ensure that labels do not propagate to backports

[Robin Westberg]

If the backport creation endpoint does not clear the labels, we can do it manually.

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Commit messages:
 - Ensure that labels do not propagate to backports

Changes: https://git.openjdk.java.net/skara/pull/965/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=965&range=00
  Stats: 24 lines in 2 files changed: 10 ins; 9 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/965.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/965/head:pull/965

PR: https://git.openjdk.java.net/skara/pull/965