Remove IS_AF_UNIX macro

Remove IS_AF_UNIX macro

The AF_UNIX macro was being used unprotected by HAVE_UNIX_SOCKETS,
apparently since 2008.  So the redirection through IS_AF_UNIX() is
apparently no longer necessary.  (More generally, all supported
platforms are now HAVE_UNIX_SOCKETS, but even if there were a new
platform in the future, it seems plausible that it would define the
AF_UNIX symbol even without kernel support.)  So remove the
IS_AF_UNIX() macro and make the code a bit more consistent.

Discussion: https://www.postgresql.org/message-id/flat/f2d26815-9832-e333-d52d-72fbc0ade896%40enterprisedb.com