Remove changing password_salt field

There's no password_salt since devise 1.2.1:

"Devise 1.2.1 does not require a password_salt column anymore if you 
are using bcrypt. If you need a kind of salt, I believe there is a 
method called authentication_salt you could use to retrieve such 
values." ~ Jose Valim (https://groups.google.com/d/msg/plataformatec-devise/saHGSLwm5U0/GCQ1PkOh8AUJ)