Remove $purpose parameter from password validity check

Remove $purpose parameter from password validity check

This was added in I56b6600 in an attempt to work around a bug in
CentralAuth, but the bug has since been fixed in a better way. No hook
functions in Gerrit use the parameter (or ever have, as far as I can
tell), and anything that was passing a value other than the default
'login' has since been removed. So let's just get rid of it instead of
keeping it around doing nothing.

Change-Id: Ie604e03d268706221161ac93eb866f477e466fb4