Bluetooth: Remove bt_storage API

The same functionality is now supported by the settings-based
solution, so remove bt_storage out of the way. There were stubs in
bt_storage to handle per-peer information (e.g. pairing keys) but this
was never actually implemented in full. The next step is to add this
support to the settings-based solution.

Leave the code for generating temporary IRK and identity address in
case BT_SETTINGS is not enabled. Also leave the code for using vendor
HCI to read the identity address, in which case the settings
implementation will not touch it.

Introduce a new bt_unpair() API to replace the removed
bt_storage_clear(), since the latter was actually doing more than just
storage management: it was also handling runtime storage of pairing
information. Later, the bt_unpair() implementation will be extended to
clear settings-based pairing storage.

There is one feature that the bt shell module looses: the ability to
give a specific identity address to the "init" command as a parameter.
We might look later in the future if this is really needed, and add a
separate API for this.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>