LDAP: proper handling of master password
    
     - copies of LDAP master password are erased as soon as
    they are not needed anymore;
     - got rid of `simple_bind_state::pw` as it was set to point
    to a variable on the stack (bug) and was not used anyway
    
    Relates: https://pagure.io/SSSD/sssd/issue/3956
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>