postgres_fdw: Obsolete comments in GetConnection()

While working on something else, I noticed $SUBJECT, which I should
have updated in commit 27e1f1456.  :-(  There are two places that need
to be updated, but in the first place the second one seemed a bit
redundant to me, because it says the same thing as the first one, and
is placed pretty close to the first one within 10 lines or so.  So I
rewrote the second one entirely into something much more simple like
the attached.

Best regards,
Etsuro Fujita