DPM: remove FDE-specific password validity persistence

On FDE devices, DevicePolicyManagerService persistently stored a flag
indicating whether the user's password is sufficient for the policy.
Since FDE is no longer supported, this code is no longer needed.

Test: atest FrameworksServicesTests:DevicePolicyManagerTest
Bug: 208476087
Change-Id: I1e2a97a354382dbee6a92b166e91ae8130f1e48c