Fix deinitializing telephony backend when it wasn't initialized

Telephony driver is now deinitialized on headset adapter driver remove so
telephony_exit should not be called on plugin exit anymore.
diff --git a/audio/manager.c b/audio/manager.c
index 12671c7..035656b 100644
--- a/audio/manager.c
+++ b/audio/manager.c
@@ -1264,10 +1264,8 @@
 	if (enabled.media)
 		btd_unregister_adapter_driver(&media_server_driver);
 
-	if (enabled.headset) {
+	if (enabled.headset)
 		btd_unregister_adapter_driver(&headset_server_driver);
-		telephony_exit();
-	}
 
 	if (enabled.gateway)
 		btd_unregister_adapter_driver(&gateway_server_driver);