`Event#remove` doesn't work with @metadata

@andrewvc found this issue when working on the http poller.

If you save any information in the `@metadata` field and try to rename the key with the mutate filter.
The filter will create the target field with a nil value and the data will remain with the `@metadata`
The mutate filter actually use the `#remove` method to remove and return the value of the key.
But the current implementation only give access to the `accessors` hash, which include all the `non-metadata` field

See:
https://github.com/elastic/logstash/blob/master/lib/logstash/event.rb#L203