removeMembership(userName) removes the specified user from a
      web project.
    Parameters
- userName
- A string representing the username of the user to remove from membership of the site.
Example
    var site = siteService.getSite("swsdp");
    if(site){
        var authorityName = "joe.user";
        site.removeMembership(authorityName);
        ...   
    }
      
    