pgsql: Remove some no-longer-needed kluges for bootstrapping, in

Log Message:
-----------
Remove some no-longer-needed kluges for bootstrapping, in particular
the AMI_OVERRIDE flag. The fact that TransactionLogFetch treats
BootstrapTransactionId as always committed is sufficient to make
bootstrap work, and getting rid of extra tests in heavily used code
paths seems like a win. The files produced by initdb are demonstrably
the same after this change.

Modified Files:
--------------
pgsql/src/backend/access/heap:
heapam.c (r1.182 -> r1.183)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.182&r2=1.183)
pgsql/src/backend/access/transam:
transam.c (r1.63 -> r1.64)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/transam.c.diff?r1=1.63&r2=1.64)
varsup.c (r1.61 -> r1.62)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/varsup.c.diff?r1=1.61&r2=1.62)
xact.c (r1.196 -> r1.197)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.196&r2=1.197)
pgsql/src/backend/utils/init:
postinit.c (r1.139 -> r1.140)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/postinit.c.diff?r1=1.139&r2=1.140)
pgsql/src/include/access:
transam.h (r1.52 -> r1.53)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/transam.h.diff?r1=1.52&r2=1.53)