tests: drop linkage to libvirt_driver_network_impl.la

The qemuxml2argvtest does not need to link to the network driver
after this commit:

  commit 0c63c117
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Fri Feb 9 15:08:53 2018 +0000

    conf: reimplement virDomainNetResolveActualType in terms of public API
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

tests/Makefile.am

@@ -583,7 +583,6 @@ qemuxml2argvtest_SOURCES = \
 	qemuxml2argvtest.c testutilsqemu.c testutilsqemu.h \
 	testutils.c testutils.h
 qemuxml2argvtest_LDADD = libqemutestdriver.la \
-	../src/libvirt_driver_network_impl.la \
 	$(LDADDS) $(LIBXML_LIBS)
 
 qemuxml2argvmock_la_SOURCES = \