pgsql: postgres_fdw: Minor cleanup for pgfdw_abort_cleanup().

postgres_fdw: Minor cleanup for pgfdw_abort_cleanup().

Commit 85c696112 introduced this function to deduplicate code in the
transaction callback functions, but the SQL command passed as an
argument to it was useless when it returned before aborting a remote
transaction using the command. Modify pgfdw_abort_cleanup() so that it
constructs the command when/if necessary, as before, removing the
argument from it. Also update comments in pgfdw_abort_cleanup() and one
of the calling functions.

Etsuro Fujita, reviewed by David Zhang.

Discussion: https://postgr.es/m/CAPmGK158hrd%3DZfXmgkmNFHivgh18e4oE2Gz151C2Q4OBDjZ08A%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/56566835039ac5eed70f188518cef1a7ea0971b2

Modified Files
--------------
contrib/postgres_fdw/connection.c | 29 +++++++++++++++--------------
1 file changed, 15 insertions(+), 14 deletions(-)