mw/core remove a dupe regression job

On merge of mediawiki core we ran two jobs:

- mediawiki-core-master-phpunit-all
- mediawiki-core-regression-master

The first was an experiment I created manually. The second is the real
one which is maintained via JJB and the one we want to keep.

Change-Id: I2aeef0bcad6389f69d94fc8b68bf49bef20b2131