Don't link pg_isolation_regress with libpq

It's not necessary and can only create confusion about which libpq
installation should be used.

Also remove some dead code from the makefile that was apparently
copied from elsewhere.

src/test/isolation/Makefile

@@ -9,12 +9,7 @@ include $(top_builddir)/src/Makefile.global
 # where to find psql for testing an existing installation
 PSQLDIR = $(bindir)
 
-ifeq ($(PORTNAME), win32)
-LDLIBS += -lws2_32
-endif
-
 override CPPFLAGS := -I$(srcdir) -I$(libpq_srcdir) -I$(srcdir)/../regress $(CPPFLAGS)
-override LDLIBS := $(libpq_pgport) $(LDLIBS)
 
 OBJS =  specparse.o isolationtester.o
 
@@ -25,12 +20,12 @@ pg_regress.o: | submake-regress
 	rm -f $@ && $(LN_S) $(top_builddir)/src/test/regress/pg_regress.o .
 
 pg_isolation_regress: isolation_main.o pg_regress.o
-	$(CC) $(CFLAGS) $^ $(libpq_pgport) $(LDFLAGS) $(LDFLAGS_EX) $(LIBS) -o $@$(X)
+	$(CC) $(CFLAGS) $^ $(LDFLAGS) $(LDFLAGS_EX) $(LIBS) -o $@$(X)
 
 all: isolationtester pg_isolation_regress
 
 isolationtester: $(OBJS) | submake-libpq submake-libpgport
-	$(CC) $(CFLAGS) $(OBJS) $(libpq_pgport) $(LDFLAGS) $(LDFLAGS_EX) $(LIBS) -o $@$(X)
+	$(CC) $(CFLAGS) $^ $(libpq_pgport) $(LDFLAGS) $(LDFLAGS_EX) $(LIBS) -o $@$(X)
 
 distprep: specparse.c specscanner.c