Kill $wgArticle

Kill $wgArticle

This has been deprecated since 1.18, throwing warnings for over a
year and a half now, and nothing uses it anywhere in core or deployed
extensions.

No extensions in SVN seem to have this anymore (we must've cleaned
up well), and the only affected extensions in Git are:

- BlueSpiceExtensions
- DynamicPageList (commented, so shouldn't be a worry)
- SemanticPageMaker
- TwitterCards

If you're the author of one of these extensions or are feeling like
cleaning up ancient code, please get them fixed.

Change-Id: I8f3a54c14be4b38a8b917e32ba3a832e0a7ca116