Do not require minimum length at login.

I want legacy users to log on to my system using ZfcUser. Among their passwords are ones with a lower number of characters than is configured by default. These are not allowed at the moment.

Of course I can (and have) adapt the input filter to remove this validator. But I guess this validation makes no sense at all. The only downside to non validating would be a performance impact.

So this PR drops it.