Remove mediawiki-phpunit-zend from mediawiki-core 'test' pipeline

It's the slowest job of all.
Usually redundant with mediawiki-phpunit-hhvm (covering the main
unit tests) and mediawiki-extenions-zend (covering MediaWiki's
setup layer working in Zend).

It'll still run from the gate pipeline, but this should speed
things up for the interim until we run tests in parallel (T50217).

Change-Id: Icac172b16bd874f1cf98649105dfbacb6a673045