Migrate mw core hhvm job to postmerge

We had the PHPUnit job under HHVM triggered on each patchset submitted.
Since that takes roughly 8+ minutes and does not provide any interesting
feedback to developers we are removing the job.

Instead create a job per branch that we will trigger after merge.

Deletes mediawiki-core-phpunit-hhvm

Creates:
mediawiki-core-regression-hhvm-REL1_19
mediawiki-core-regression-hhvm-REL1_20
mediawiki-core-regression-hhvm-REL1_21
mediawiki-core-regression-hhvm-REL1_22
mediawiki-core-regression-hhvm-REL1_23
mediawiki-core-regression-hhvm-master

Although only REL1_23 and master will be triggered by Zuul.

Bug: 64687
Change-Id: I96d7a20c1b9dd2e5fe7dbd270ce60e8c31263318