adb: call run_transport_disconnects() only once.

Currently run_transport_disconnects() are called twice. One is in
handle_offline(), another is before destroying transport.
The users of disconnect callback are listener, adb_auth_client, and
remote_sockets. All of them need only to be called once. And after
handle_offline, no new listeners, adb_auth_client, or remote_sockets
can be connected to the offlined transport. So I think we can remove
the second call to run_transport_disconnects().

Change-Id: I1ef8b6b7b5ab7ae1bad109be107c85973d65a2e3