Bluetooth: host: Avoid removing keys from controller that are not added

Avoid removing identity keys from the controller during the pairing
procedure. During the pairing procedure the keys will be cleared before
they are updated. This causes an unnecessary warning from HCI core where
it tries to remove an IRK key-set from the controller that has has not
been added yet.

While this is not an issue, the warning from HCI core is misleading and
might lead to unnecessary questions and investigations.
Warning appeared after: 6c6bd8c4

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>