Move VisualEditor disabling from now-scrapped preferences section

'editing/beta' section stopped being used by anything other than VisualEditor
in Id8b9a47a in June 2013, and the section was removed from MediaWiki core in
I76667a15cc today.

Change-Id: Ida16f714c28c46d552b92c05b07b63f5d6dd361e
Merge "Move VisualEditor disabling from now-scrapped preferences section"