Fix creating non-parameterized hashes in ParameterizedPassword

I noticed MWOldPassword is broken while working on
I7024b287a7. When generating new passwords for it,
a superfluous : is being added to the serialized hash
within the database (and that breaks parsing so that
people can't ever log in).

As this is not really relevant in the real world (as
nobody is hopefully using plain MD5 passwords anymore),
this doesn't need any backward compatibility handling
for the broken hashes.

Change-Id: I753c135a6de39008488bd7462c2bfcda2cbac116