User sync should clean attributes removed from ldap

If an attribute from the LDAP is removed, the corresponding user profile attribute is not cleaned up.

I have seen an actual use case where users of the organization might loose their email address while still being able to log in.

Steps to reproduce:

1. give a test user an email, e.g. by the following ldif:
    

   dn: CN=User101,CN=Users,DC=localhost,DC=nodomain
   changetype: modify
   replace: mail
   mail: user101@localhost.nodomain
   

2. log in as that user - user gets email set in their profile
3. remove the email attribute in LDAP:

   dn: CN=User101,CN=Users,DC=localhost,DC=nodomain
   changetype: modify
   delete: mail
   

4. log in again with that user

Expected behavior:

• user has no loger their e-mail address set in the profile

Observed behavior:

• email is still set in the user profile