pgsql: Remove the no-longer-useful HashItem/HashItemData level of

Log Message:
-----------
Remove the no-longer-useful HashItem/HashItemData level of structure.
Same motivation as for BTItem.

Modified Files:
--------------
    pgsql/src/backend/access/hash:
        hash.c (r1.82 -> r1.83)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hash.c.diff?r1=1.82&r2=1.83)
        hashinsert.c (r1.40 -> r1.41)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashinsert.c.diff?r1=1.40&r2=1.41)
        hashovfl.c (r1.49 -> r1.50)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashovfl.c.diff?r1=1.49&r2=1.50)
        hashpage.c (r1.54 -> r1.55)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashpage.c.diff?r1=1.54&r2=1.55)
        hashsearch.c (r1.42 -> r1.43)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashsearch.c.diff?r1=1.42&r2=1.43)
        hashutil.c (r1.45 -> r1.46)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/hash/hashutil.c.diff?r1=1.45&r2=1.46)
    pgsql/src/include/access:
        hash.h (r1.64 -> r1.65)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/hash.h.diff?r1=1.64&r2=1.65)