Remove precedence hacks no longer needed without postfix operators.
It's no longer necessary to assign explicit precedences to GENERATED,
NULL_P, PRESERVE, or STRIP_P.
Actually, we don't need to assign precedence to IDENT either; that was
really just there to govern the behavior of target_el's "a_expr IDENT"
production, which no longer ends with that terminal. However, it seems
like a good idea to continue to do so, because it provides a reference
point for a precedence level that we can assign to other unreserved
keywords that lack a natural precedence level.
Research by Peter Eisentraut and John Naylor; comment rewrite by me.
Discussion: https://postgr.es/m/
38ca86db-42ab-9b48-2902-
337a0d6b8311@2ndquadrant.com