Revert "Don't allow deleting last remaining fingerprint"

This reverts commit bee2fddc.

Not allowing that is both confusing to the user and inconsistent:
0 enrolled fingerprints (and thus fingerprint authentication being
not used despite it being enabled) is the state right after enabling
fingerprint authentication, thus it seems misguided to prevent the
user from returning to that state.

Change-Id: I1d4b5dd9d55f17f086017503f5d79ebcd8b15d24