Swagger: Don't show derived parameters in param lists.

When using the `http` configuration for a param to a remote method, the param is still shown in the API explorer.

For example, I might use a derived param to get the current userID:

```javascript
accepts.push({arg: 'userId', type: 'number', http: function(ctx) {
  return ctx.req.accessToken && ctx.req.accessToken.userId;
}});
```

That way, when the callback to `loopback.remoteMethod()` is called, it has the current userId (since we are not passed the `ctx` object, we have no way to calculate it.

These sorts of params should not show up in the API explorer as they are not accepted by the user, they are derived from the `ctx`.