iface-modem-signal: remove unneeded skeleton clear attempt

It won't do anything because clear_values() tries to get the skeleton
from the modem object, and the skeleton hasn't been set at that point.

src/mm-iface-modem-signal.c

@@ -536,7 +536,6 @@ mm_iface_modem_signal_initialize (MMIfaceModemSignal *self,
                   NULL);
     if (!skeleton) {
         skeleton = mm_gdbus_modem_signal_skeleton_new ();
-        clear_values (self);
         g_object_set (self,
                       MM_IFACE_MODEM_SIGNAL_DBUS_SKELETON, skeleton,
                       NULL);