pgsql: Change WorkTableScan to not support backward scan.

Log Message:
-----------
Change WorkTableScan to not support backward scan.  The apparent support
didn't actually work, because nodeRecursiveunion.c creates the underlying
tuplestore with backward scan disabled; which is a decision that we shouldn't
reverse because of performance cost.  We could imagine adding signaling from
WorkTableScan to RecursiveUnion about whether backward scan is needed ...
but in practice it'd be a waste of effort, because there simply isn't any
current or plausible future scenario where WorkTableScan would be called on
to scan backward.  So just dike out the code that claims to support it.

Modified Files:
--------------
    pgsql/src/backend/executor:
        execAmi.c (r1.100 -> r1.101)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execAmi.c?r1=1.100&r2=1.101)
        nodeWorktablescan.c (r1.3 -> r1.4)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeWorktablescan.c?r1=1.3&r2=1.4)