Reverting r39619 "(bug 13689) Page output can be suppressed on Article::purge() by...

Reverting r39619 "(bug 13689) Page output can be suppressed on Article::purge() by passing bool false to it. Default (ie: nearly 100+% of the time) is true."

Article::purge() is a UI action entry point, not a backend method. Backend-only parameters aren't appropriate here; instead a method suitable for backend usage should be created (and/or in an ideal world, we should finish splitting Article backend stuff from UI frontend as separate classes)