authsecret: remove factoryReset()

Factory reset will be handled by each device from recovery or in the
bootloader in response to `fastboot -w`. The requirements of the factory
reset are made explicit. As a result, VTS tests are moving to a host
side test that can factory reset between each test case.

Also clarifies when the remaining method is called.

Test: AuthSecretHidlTest
Bug: 71527305
Change-Id: I9a29568e022eb83061d8db68e1e7971fc53bd823