Fix interpretation of "A-type" password hashes

Fix interpretation of "A-type" password hashes

An A-type hash is an unsalted hash. A B-type hash is a salted hash of
the form md5(salt "-" md5(password)). So it's not correct to have an
A-type hash with a salt. User::comparePasswords() and
CentralAuthUser::getPasswordFromString() already get this right, they
generate :B: prefixes for legacy salted hashes where the salt is not
specified in the database.

Change-Id: Icb809274f9f63641e54daf98332a5646fd58b550