highlighter: exec_separator will re-create '_to_' groups

so we don't need to re-create them in the main for loop of
airline#highlighter#highlight() and so skip those groups.

autoload/airline/highlighter.vim

@@ -259,6 +259,9 @@ function! airline#highlighter#highlight(modes, ...)
           if bnr > 0 && index(buffers_in_tabpage, bnr) == -1
             continue
           endif
+        elseif name =~# '_to_'
+          " group will be redefined below at exec_separator
+          continue
         endif
         if s:group_not_done(airline_grouplist, name.suffix)
           call airline#highlighter#exec(name.suffix, mode_colors)