Remove obsolete replacement system() on darwin

Per comment in the file, this was fixed around OS X 10.2.

Author: petere

src/backend/port/Makefile

@@ -23,9 +23,6 @@ include $(top_builddir)/src/Makefile.global
 
 OBJS = atomics.o dynloader.o pg_sema.o pg_shmem.o $(TAS)
 
-ifeq ($(PORTNAME), darwin)
-SUBDIRS += darwin
-endif
 ifeq ($(PORTNAME), win32)
 SUBDIRS += win32
 endif
@@ -44,5 +41,4 @@ endif
 
 distclean clean:
 	rm -f tas_cpp.s
-	$(MAKE) -C darwin clean
 	$(MAKE) -C win32 clean