Drop the constraint on user email uniqueness

We received the following error message when trying to create a new user with the email address that belong to other user in FishEye.

Your request was unable to be processed
org.hibernate.NonUniqueResultException - query did not return a unique result: 5

You are running release FE3.0.1 (20130606055351 2013-06-06), please report your release number when reporting bugs.

This could be an issue for us because the user list in FishEye will most likely sourced from JIRA and we have created some users with the same email address (it is possible to create different users with the same email address there)