pgsql: Remove dead push/pop rollback code.

Log Message:
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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)