Clean up VisualEditor settings

Remove wmgVisualEditorHide (we don't need this any more) and allow
editing for NS_MAIN and NS_USER on mediawikiwiki (this had been
over-looked in the December deployment to enwiki).

Bug: 42936
Change-Id: Iec91b30422017ce9d71bc71b2cbeb71688e0d18f
Merge "Clean up VisualEditor settings"