LPS-31791 Site Administrator role can remove a user with Site Owner role from Site

Hi @JorgeFerrer,

Three things to take into account in this pull request:
- It's not necessary to modify the remove role pages because this restriction is already covered by the method UsersAdminImpl.filterGroupRoles which it's used in:
select_organization_role.jsp, 
select_site_role.jsp, 
edit_site_assignments_user_groups_roles.jsp, 
edit_site_assignments_users_roles.jsp, 
edit_user_group_roles_role.jsp, 
edit_user_roles_role.jsp

and it works in the same way as our new filters

- I have realized that we check the UPDATE permission over the users to remove in UserOrganizationChecker but however we check the ASSIGN_MEMBERS permission in the service, is this correct?

- We can prepare some unit tests after the pull request approval.

Thanks.