Remove the feature/stretch Jenkins jobs

They won’t be useful once #12536 (closed) is completed, but I’ll only do that once anonym tells me he doesn’t need the results of the test jobs anymore (I guess they may be useful while working on the test suite on Stretch).

Related issues

Blocked by #12536 (closed)
Blocked by #12288 (closed)
Blocks #13233 (closed)

Original created by @intrigeri on 12604 (Redmine)

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