Fix gatt autoconnect

When BLE connection is complete, stack would inform connection_manager by invoking on_connection_complete. When shim was added, it started using remove_unconditional_from_shim.

The difference between those two methods is that on_connection_complete would keep information about background connections, and remove_unconditional_from_shim would just erase everything.

Information about background connection is used after connection is complete in gatt_send_conn_cback, to mark apps as using link, by invoking gatt_update_app_use_link_flag. When remove_unconditional_from_shim is used, this is not done, and app would not be able to reconnect.

Test: manual
Bug: 232258524
Change-Id: Ie2fb572ac2e0d4124f603c4d5c440de69da82d0c