Revert "EJB UserDAO : getUserByMail return a list"

This reverts commit 4d7960458e78e6b80197fca7e4d04c2758975578.
Revert "EJB UserDAO : methode insert to insert a new User"

This reverts commit f1b9d5343f810f26d246230700aefa117fb59b49.
Fix ControllerUser after the two reverts

EJB exceptions are handled by the controller.

When creating a new user, it insert the tuple in the database. If
the mail if already used, it throwse an SQL exception, catch by the
EJB container, which throws an EJB excpetion catched by the
controllerUser.

The same mechanism takes place for the login, when no the mail is
not in the database.