Remove the feature/stretch Jenkins jobs

Originally created by @intrigeri on #12604 (Redmine)

They won’t be useful once tails/tails#12536 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 tails/tails#12536
• Blocked by tails/tails#12288
• Blocks tails/sysadmin#13233