Force all devices to migrate to synthetic password

Remove the property used to disable the migration.
With this change disabling of synthetic password will
no longer work.

https://partnerissuetracker.corp.google.com/issues/146040259
has been created to track the removal of all non-synthetic
password related flows.

Test: Device boot and validate FBE (inclding OTA).

Change-Id: Ib4633ff2f667ee7f88195c7cbd07ba99eea76d51