Remove unneeded syntax.off() call

syntax.on() is no longer called anywhere in the package.

src/packages/spell-check/lib/spell-check.coffee

@@ -12,6 +12,3 @@ module.exports =
     rootView.eachEditor (editor) ->
       if editor.attached and not editor.mini
         editor.underlayer.append(new SpellCheckView(editor))
-
-  deactivate: ->
-    syntax.off '.spell-check'