connectivity: remove sync in device announcement

It is unnecessary because the device performs synchronization
during the bootstrap. Moreover, it could result in the
failure to receive notifications on iOS because iOS does not
respond to sync connections in the background. However, the
other side may consider it as already connected.

jami-client-ios#281

Change-Id: Id55e7bfaafba1e45a6c27afdebf682c33acd9d30