Fix crash in virsystemdtest with dbus 1.7.6

D-bus introduced some changes in its locking code. Overriding the init
function skips the new locking init and thus crashes later in libvirt
test. Removing the function makes the test pass again.

(cherry picked from commit 5e397d9c)

tests/virsystemdmock.c

@@ -31,11 +31,6 @@ void dbus_connection_set_change_sigpipe(dbus_bool_t will_modify_sigpipe ATTRIBUT
 {
 }
 
-dbus_bool_t dbus_threads_init_default(void)
-{
-    return 1;
-}
-
 DBusConnection *dbus_bus_get(DBusBusType type ATTRIBUTE_UNUSED,
                              DBusError *error ATTRIBUTE_UNUSED)
 {