pgsql: Remove configure probe for net/if.h.

Remove configure probe for net/if.h.

<net/if.h> is in SUSv3 and all targeted Unixes have it.  It's used in a
region that is already ifdef'd out for Windows.  We're not using it for
any standard definitions, but it's where AIX defines conventional socket
ioctl numbers.

Discussion: https://postgr.es/m/CA+hUKGKErNfhmvb_H0UprEmp4LPzGN06yR2_0tYikjzB-2ECMw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2f8d918359ccb34533690735eb46b851002ac698

Modified Files
--------------
configure                  | 2 +-
configure.ac               | 1 -
src/backend/libpq/ifaddr.c | 3 ---
src/include/pg_config.h.in | 3 ---
src/tools/msvc/Solution.pm | 1 -
5 files changed, 1 insertion(+), 9 deletions(-)