siptransport: do not force shutdown

Following the pjsip doc, pjsip_transport_dec_ref will shutdown the
transport if necessary. Moreover, when connectivity changes, the
transport is already with the state PJSIP_TP_STATE_SHUTDOWN.
If deleteTransport calls pjsip_transport_shutdown, the transport will
still be used by the sipaccount, but getUDPTransport will return a
unusable transport.

Change-Id: I25b7f0d282d62d28b7a49065d72e0c0de620b310