Clean up of postgresql_fdw.c

Hi all,

I am looking at postgresql_fdw.c and I am cleaning up the functions inside
it.
Please find attached a patch that removes is_immutable_func as it does
exactly the same thing as func_volatile in lsyscache.c.
There is still one function remaining in postgresql_fdw.c called
pgxc_is_expr_shippable that is related to FQS planner.
Ashutosh, any thoughts about where to put it?
--
Michael Paquier
http://michael.otacoo.com