Remove base-zero pattern migration in LockSettingsService

Base-zero pattern was a bug introduced during M and later fixed before
M got released (b/21433955#comment23). As a result there is no need to
keep this logic around after M.

Bug: 63619579
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/locksettings
Change-Id: I9c14f86783247a3cc0db20447b5ec0622785a28b