Prevent multiple register of service change indication

Use Case:
Pair DUT to a remote LE device. The host registers for the service
change indication twice, once when the connection callback is
received and another time, when the SMP process completes

Failure:
There are a few issues caused by this.
First, the duplicate action of service change indication registeration
is unnecessary.
The registeration that follows the SMP process adds a gatt_if to the
apps holding the current link. This gatt_if (=1) never disconnects
after registeration failure or success. And hence the host can never
physically disconnect the link.

Fix:
This change removes the service change registeration at SMP
process completion

Test: manual
Change-Id: I0d7566e2b4a9d01aa7926cdbe9f528c3941fcfeb