Remove length validation from signin password field

This small change removes the signin password field length validation to allow legacy users with abnormal length passwords to sign in.

The validation remains in place for registration and password reset.

![6u2xad6](https://f.cloud.github.com/assets/1170410/2229155/1151d262-9ae8-11e3-8ce7-f3d5f12b14b1.gif)