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 - a workaround since we are not passed the `ctx` object in that callback.

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`.