Delete slots as the listeners are unregistered.

For the current code this is a no-op because the new
slots are always computed immediately in the same
critical section.

When the code uses callbacks for the carrier service UID
though, there will no longer be a time where the entire
array is reset. Instead, registering the callback will
immediately trigger the call that populates it again,
which means the value needs to be removed when a SIM
card is removed. Unregistering the callback is a good
time to do this.

Test: CarrierPrivilegeAuthenticatorTest
Change-Id: I6de4abdc57ffa455d7f8e4d35f5dd1e18937e94e