Remove obsolete locksettings db migrations

Remove migration code that was only needed on devices upgrading to much
earlier versions of Android:

- "migrated" and "migrated_user_specific" were only needed for upgrades
  to Android K (http://ag/297566)

- "migrated_biometric_weak" was only needed for upgrades
  to Android M (http://ag/609529)

- "migrated_lockscreen_disabled" was only needed for upgrades
  to Android M (http://ag/677350).

- "migrated_wear_lockscreen_disabled" was only needed for upgrades
  to Android O (http://ag/1595971).

Leave the "migrated_keystore_namespace" one for now, as it's
significantly more recent (needed for upgrades to Android S).

Change-Id: Ia69b8baf1995dd6deba8cf58440a481542c67bba