Fix action name to delete onion via API

Sending a POST-request to https://www.onions.io/api/deleteOnion with a valid SessionKey, ends in a 404 error page. I think this is caused by a conflict in naming the action method.

### config/routes.rb
````ruby
post '/api/deleteOnion' => 'onions#deleteOnion'
````

### app/controllers/onion_controller.rb
````ruby
def delete_onion
  # ...
end
````