Fixed return value (coscine/issues#1529)

src/Project/Controllers/ProjectController.cs

@@ -757,7 +757,7 @@ namespace Coscine.Api.Project.Controllers
 
             _invitationModel.Delete(invitation);
 
-            return Ok($"User {user.Id} is now {role.DisplayName} of project {project.Id}.");
+            return NoContent();
         }
 
         private static bool IsValidEmail(string email)