[FIX] auth_ldap: One failed login attempt counts twice

#### Description of the error
There is a problem in the latest version of the Odoo auth_ldap module (Authentication via LDAP). When an LDAP user tries to log into a running Odoo instance and when he/she misspells the password, the method users_ldap.CompanyLDAP.authenticate is called twice from within users_ldap.users.login.

When the LDAP server records the number of failed attempts, this problem divides the number of possible attempts by two!
#### Scenario to reproduce the error
1. Setup a test environment using the auth_ldap module.
2. Run Odoo in a debugger and set a breakpoint in the method users_ldap.CompanyLDAP.authenticate4
3. Go to the login screen of Odoo.
4. Try to log-in using an LDAP user and an erroneous password.
5. Verify in the debugger that the method users_ldap.CompanyLDAP.authenticate is called twice.

#### Suggested improvement
The method users_ldap.users.login seems to be superfluous. If that method is not redefined and if wrong credentials are provided in the login screen, the authenticate method is only called once.