ui-out.c: Remove unused enum

This is unused since 54eb231c4b, where
static arrays of ui_out_levels were replaced with vectors.

gdb/ChangeLog:

	* ui-out.c (MAX_UI_OUT_LEVELS): Remove.

gdb/ui-out.c

@@ -37,12 +37,6 @@ struct ui_out_hdr
     struct ui_out_hdr *next;
   };
 
-/* Maintain a stack so that the info applicable to the inner most list
-   is always available.  Stack/nested level 0 is reserved for the
-   top-level result.  */
-
-enum { MAX_UI_OUT_LEVELS = 8 };
-
 struct ui_out_level
   {
     /* Count each field; the first element is for non-list fields.  */