Bluetooth: Fix clearing conn->le.keys when unpairing

When calling bt_unpair() the keys were removed from flash, however a
pointer was left to the invalidated keys in case there was an existing
connection. This would then lead to a bogus entry being stored in
flash for a zero-address peer device. Fix this issue by clearing the
conn->le.keys pointer in the bt_unpair() function.

Fixes #15325

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