Remove ecosystem link from guides

Right now, there's an [ecosystem link](http://rubyonrails.org/ecosystem) in the header of [the guides pages](http://guides.rubyonrails.org):


Unfortunately the ecosystem page no longer exists on rubyonrails.org.

This PR removes that dead link.



_NOTES:_

I realise that this is a docrails change, but I wanted to double check first that there isn't a new, or better, page for ecosystem out there.