Simplify logic on if statement

Referencing the same boolean multiple times in a logical statement is
probably not what was intended.

plugin/airline.vim

@@ -27,8 +27,7 @@ function! s:init()
 endfunction
 
 function! s:on_window_changed()
-  if pumvisible() && (!&previewwindow ||
-        \ (g:airline_exclude_preview && &previewwindow))
+  if pumvisible() && (!&previewwindow || g:airline_exclude_preview)
     return
   endif
   call s:init()