[XQuery] IBM-XQ-017: Delete error XP0018

(IBM-XQ-017) Section 3.1.5, Function Calls: Error XP0018, referenced in 
this section, is just a special case of Error XP0002, which is used in 
several other places for the same condition. Since Error XP0002 is more 
general-purpose, we should eliminate XP0018 and change all its references 
to XP0002. (If retained, XP0018 should be made dynamic rather than 
static.)

--Don Chamberlin