Fire AirlineToggledOff autocmd even if airline is disabled

Fixes #2377.  This regression was introduced in 2e9df43.

autoload/airline/util.vim

@@ -174,7 +174,7 @@ function! airline#util#has_custom_scm()
 endfunction
 
 function! airline#util#doautocmd(event)
-  if !exists('#airline')
+  if !exists('#airline') && a:event !=? 'AirlineToggledOff'
     " airline disabled
     return
   endif