UserCredentialsSettings: remove unneeded checks for locksettings keys

Since Android 12 (frameworks/base change
Iac339e64812232875afa9dc6109b848b50b0ef2f), the locksettings keys are in
their own key namespace.  Therefore, they are no longer included when
UserCredentialsSettings enumerates the keys in the app namespace, so
there is no longer any need to explicitly ignore these keys.

This is a cleanup that is a prerequisite for a cleanup in locksettings
where LockPatternUtils.SYNTHETIC_PASSWORD_KEY_PREFIX will be renamed and
have its visibility reduced.  No change in behavior intended.

Test: Added temporary logging of all key aliases enumerated by
      keyStore.aliases() in getCredentialsForUid().  Set an LSKF, and
      went to the User Credentials page in the Settings app.  Verified
      via logcat that the locksettings keys aren't being enumerated.
Change-Id: If9040b55a5743c9b0e174538f67c669e864f0312