Fix a bug that call clearing tones are not started when two other call parties hangup

Call scenario
- DUT calls A and the call is answered
- B calls DUT
- Press R to accept B
- A hangs up and then B hangs up There are no call clearing tones after a brief beep.

Cause
handle_dialtone_timeout() calls chan_voicemngr_get_active_subchannel() to get the sub-channel
and the sub[0] is returned and its state is ONHOOK. So the call clearing procedure is not
started by handle_dialtone_timeout().

Solution
Let chan_voicemngr_get_active_subchannel() select the sub-channel with state CALLENDED over
ONHOOK.

Also remove the unused call state ANSWER.