call connection_manager::on_connection_complete from just one place

Currently, on_connection_complete is called from three different
locations. It's enough to call it from just one place.

on_connection_complete marks apps doing direct connection as no longer
interested in particular device. It should be called after TCB is
created, and interested apps are rewritten to gatt control blocks.
Therefore, just the latest call in chain is left, in gatt_send_conn_cback.

This is needed for next patch in chain, which is removing the doubled
list of registrations from GATT.

Test: atest avatar:'LeHostTest#test_disconnect' -v
Bug: 372202918
Flag: EXEMPT, removing redundant code
Change-Id: Ibd2ff062dc26df7c2c2a627328d97e2e9da3551d