Remove support of legacy credential hash

CDD started to require hardware-based lockscreen authentication
(gatekeeper) since Android N. Support of legacy credential hash
(lockscreen password hashed directly on device) is now totally
obsolete and can be removed safely, provided that there is no
need to support an OTA from M (or before) straight to R.

Bug: 63619579
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/locksettings
Change-Id: Ie71fec3eb0eaba8ec4c8e33e791a3d553d9355f9