Disable contrib/postgres_fdw

postgres_fdw was enabled by mistake in c9d4c1e5, but it
should remain disabled as it's still undergoing work in order to
function properly in for Greenplum.

Discussion: https://groups.google.com/a/greenplum.org/forum/#!topic/gpdb-dev/Lepem3qwJGw

contrib/Makefile

@@ -42,7 +42,6 @@ SUBDIRS = \
 		pgrowlocks	\
 		pgstattuple	\
 		pg_visibility	\
-		postgres_fdw	\
 		seg		\
 		spi		\
 		tablefunc	\