Remove legacy mediawiki core jobs

Superseeded by mediawiki-vendor-integration which supports
mediawiki/vendor and runs all PHPUnit tests.

Removes:

'{name}-install-{databasetype}'
'{name}-phpunit-{phpunit-group}'

Jobs are no more triggered since Zuul configuration change
https://gerrit.wikimedia.org/r/#/c/163817/

Change-Id: Idf44bb9e6256818660cb04f090cf72dcbde0f421