Remove rails.png.

No reason to have a file, we've got data-URIs now! It didn't make sense
to me that you no longer have to remove public/index.html, but you still
need to remove rails.png. This means that you won't have to any more
when making a new Rails app.

Now, a bunch of Railties tests fail because we use the fact that we know that rails.png exists to test them. I wanted to submit this as a PR to get feedback about the idea before I spent the time figuring out how to deal with that.

/cc @dhh