Cleanup some -I$(libpq_srcdir) in makefiles

Hi all,

While looking at a patch from David, I have noticed $subject:
https://www.postgresql.org/message-id/CAApHDvpgB+vxk=W6OPKidwzZEo6kniFQidNoMzR8P4ROtyky2w@mail.gmail.com

adminpack and old_snapshot have no need for those references as they
don't use libpq.  Any objections to a small-ish cleanup that removes
those references, as per the attached?

Thanks,
--
Michael