pgsql: Remove Windows-specific definition of S_ISDIR().

[tgl(at)postgresql(dot)org (Tom Lane)]

Log Message:
-----------
Remove Windows-specific definition of S_ISDIR().  This should not be here;
if there are any Windows configurations where port/win32.h fails to
provide the macro, it should be fixed in the latter file not here.

Modified Files:
--------------
    pgsql/src/bin/psql:
        copy.c (r1.79 -> r1.80)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/copy.c?r1=1.79&r2=1.80)