pgsql: Remove the stub support we had for UNION JOIN; per discussion,

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/src/backend/optimizer/path:
        clausesel.c (r1.78 -> r1.79)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/clausesel.c.diff?r1=1.78&r2=1.79)
    pgsql/src/backend/optimizer/plan:
        initsplan.c (r1.115 -> r1.116)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/initsplan.c.diff?r1=1.115&r2=1.116)
    pgsql/src/backend/optimizer/prep:
        prepjointree.c (r1.36 -> r1.37)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepjointree.c.diff?r1=1.36&r2=1.37)
    pgsql/src/backend/parser:
        gram.y (r2.533 -> r2.534)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.533&r2=2.534)
        Makefile (r1.42 -> r1.43)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/Makefile.diff?r1=1.42&r2=1.43)
        parser.c (r1.64 -> r1.65)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parser.c.diff?r1=1.64&r2=1.65)
        scan.l (r1.131 -> r1.132)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/scan.l.diff?r1=1.131&r2=1.132)
    pgsql/src/backend/utils/adt:
        ruleutils.c (r1.213 -> r1.214)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c.diff?r1=1.213&r2=1.214)
    pgsql/src/include/nodes:
        nodes.h (r1.183 -> r1.184)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/nodes.h.diff?r1=1.183&r2=1.184)
    pgsql/src/include/parser:
        gramparse.h (r1.32 -> r1.33)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/gramparse.h.diff?r1=1.32&r2=1.33)
    pgsql/src/test/regress/expected:
        join.out (r1.25 -> r1.26)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/join.out.diff?r1=1.25&r2=1.26)
        join_1.out (r1.6 -> r1.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/join_1.out.diff?r1=1.6&r2=1.7)
    pgsql/src/test/regress/sql:
        join.sql (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/join.sql.diff?r1=1.17&r2=1.18)
    pgsql/src/interfaces/ecpg/preproc:
        preproc.y (r1.320 -> r1.321)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/preproc/preproc.y.diff?r1=1.320&r2=1.321)