LockSettingsService: remove TODO for getCredentialTypeInternal()

The TODO for getCredentialTypeInternal() was added by ag/9569544
("Introduce Lockscreen PIN credential as first-class citizen"), which
predated ag/16692269 ("Add a binder cache to
ILockSettings#getCredentialType").  As the latter change effectively
addressed the TODO, remove the TODO.

Test: N/A
Change-Id: Ia9a33053f9e0db030bfd4272c2c4318278f896a4