RFR: 943: remote: GitLab: You can only delete protected branches using the web interface

[Robin Westberg]

Use a rest api to delete branches, to allow dependent pr branches to be configured as protected.

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Commit messages:
 - Use a rest api to delete branches

Changes: https://git.openjdk.java.net/skara/pull/1105/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1105&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-943
  Stats: 45 lines in 7 files changed: 37 ins; 7 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1105.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1105/head:pull/1105

PR: https://git.openjdk.java.net/skara/pull/1105