Remove server instance existence check logic in disconnected case

Remove server instance existence check logic in disconnected case

If stop-adv request is comming before disconnected, event if
server_instance is removed, gatt_client_info_list is not removed.

This issue causes that CONNECTED event is not occured on below scenario:
Start-Adv > Connected > [Stop-Adv > Disconnected] > Start-Adv > Connected
                       ([Disconnected > Stop-Adv] is no problem.)

In last connected step, server_instance and gatt_client_info_list are
exsiting. So, connected event is not delivered to service layer.

Change-Id: I940b3e0e6d75c6ad4315fcac40f1af6f3ef612ee