Update dummy CREATE ASSERTION grammar

Update dummy CREATE ASSERTION grammar

While we are probably still far away from fully implementing
assertions, all patch proposals appear to take issue with the existing
dummy grammar CREATE/DROP ASSERTION productions, so update those a
little bit.  Rename the rule, use any_name instead of name, and remove
some unused code.  Also remove the production for DROP ASSERTION,
since that would most likely be handled via the generic DROP support.

extracted from a patch by Joe Wildish