row_number() over () window function can be used without any column in the OVER
clause. Additionally, the item doesn't reference any tables, as it's not
effectively referencing any table. Rather it is specifically built based
on the end temporary table used for window function computation.

This caused remove_const function to wrongly drop it from the ORDER
list. Effectively, we shouldn't be dropping any window function from the
ORDER clause, so adjust remove_const to account for that.

Reviewed by: Sergei Petrunia sergey@mariadb.com