pgsql: Remove dead push/pop rollback code.

Log Message:
-----------
Remove dead push/pop rollback code.  Vadim once planned to implement
transaction rollback via UNDO but I think that's highly unlikely to
happen, so we may as well remove the stubs.  (Someday we ought to
rip out the stub xxx_undo routines, too.)  Per Alvaro.

Modified Files:
--------------
    pgsql/src/backend/access/heap:
        heapam.c (r1.185 -> r1.186)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.185&r2=1.186)
    pgsql/src/backend/access/transam:
        xact.c (r1.197 -> r1.198)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.197&r2=1.198)
    pgsql/src/include/access:
        htup.h (r1.72 -> r1.73)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/htup.h.diff?r1=1.72&r2=1.73)
        xact.h (r1.74 -> r1.75)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xact.h.diff?r1=1.74&r2=1.75)