cleanup the custom block chapter

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed Tickets | -

This was showing incomplete code. Its still less than awesome, as the base sonata bundle provides an RSS block, and the Cmf BlockBundle also provides one but it works somehow different (and its not profiting from the fact that you could store data like the rss source in the block, making it rather pointless)

Still i think this is an improvement over the previous code as its at least working.