Move VisualEditor disabling from now-scrapped preferences section

'editing/beta' section stopped being used by anything other than
VisualEditor in 3c0aa26a0da68a9 in June 2013, and the section was
removed from MediaWiki core in ief2c7d1df876fb04.

This is a follow-up to 799e11ca7e3daa7 which omitted one occurrence.

Change-Id: I05a6cce89d71808d82c12f54beb6b7a6730b265f