Remove the stub support we had for UNION JOIN; per discussion, this is

not likely ever to be implemented seeing it's been removed from SQL2003.
This allows getting rid of the 'filter' version of yylex() that we had in
parser.c, which should save at least a few microseconds in parsing.

src/test/regress/sql/join.sql

@@ -198,10 +198,6 @@ SELECT '' AS "xxx", *
 -- More complicated constructs
 --
 
--- UNION JOIN isn't implemented yet
-SELECT '' AS "xxx", *
-  FROM J1_TBL UNION JOIN J2_TBL;
-
 --
 -- Multiway full join
 --