Remove CredentialHash serialization

They were added to support lockscreen FRP on a device
with Gatekeeper credential. However that feature
was removed before shipping (ag/2527709), so these
serialization routines are no longer relavent.

Bug: 36814845
Bug: 63619579
Test: atest com.android.server.locksettings
Change-Id: I69be4beeab61b5fcf4b32c52251591729f3da4f3