Revert "make mediawiki jobs runnable on slaves"

--references with http backend does not really works with the huge
repository mediawiki/core. Need to investigate.

This reverts commit 1b9ee3dc64937d1a23d278ad564a0e4fc5ba31f4.

Change-Id: I13e6d297f91745fca48f26516c85f0493d735fb5