[Telephony] remove the RuntimeException thrown

Bug: 196880723
Test: ATP apct/device_boot_health_check_extra
      TMUS-Repeated test to unregistermmtel after registration.
          Volte dials/disconnects, airplane mode on/off,
          Sim removed, power on/off

Change-Id: I050d4affd403604bdae21ef3bc7f9ddf93ffa9e4

telephony/java/android/telephony/ims/ImsMmTelManager.java

@@ -619,9 +619,9 @@ public class ImsMmTelManager implements RegistrationManager {
 
         ITelephony iTelephony = getITelephony();
         if (iTelephony == null) {
-            throw new RuntimeException("Could not find Telephony Service.");
+            Log.w("ImsMmTelManager", "Could not find Telephony Service.");
+            return;
         }
-
         try {
             iTelephony.unregisterMmTelCapabilityCallback(mSubId, c.getBinder());
         } catch (RemoteException e) {