pgsql: Minor code cleanup: ExecHash() was returning a null

Log Message:
-----------
Minor code cleanup: ExecHash() was returning a null TupleTableSlot, and an
old comment in the code claimed that this was necessary. Since it is not
actually necessary any more, it is clearer to remove the comment and
just return NULL instead -- the return value of ExecHash() is not used.

Modified Files:
--------------
    pgsql/src/backend/executor:
        nodeHash.c (r1.91 -> r1.92)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeHash.c.diff?r1=1.91&r2=1.92)
        nodeHashjoin.c (r1.69 -> r1.70)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/nodeHashjoin.c.diff?r1=1.69&r2=1.70)