Revert r77555 and followups r77563, r77572, r78116 (merge DisableAccount to core).

Revert r77555 and followups r77563, r77572, r78116 (merge DisableAccount to core).

Per CR at the time: this creates a nearly irreversable action that is not nearly well documented enough (even if disabled by default).

We already have the $wgBlockDisablesLogin kludge in place for this. If we're going to do more work on this idea, it should be well thought out, not another hack.