Remove unused code

[franmomu]

Subject

I was going to fix the config deprecation in this bundle, but it looks like neither the Configuration class nor the Extension class are used, there are no services declared, so I guess it is better to just remove them.

I am targeting this branch, because is the one.

Changelog

### Removed
- Removed useless `Configuration` and `SonataDatagridExtension` class

[core23]

We can't do this on a stable branch

[OskarStark]

Yep it would be a BC break, this is not possible, please provide the patch for the deprecation here too.
Thanks

[franmomu]

Ok, I will. But why is this BC? I thought it wouldn't because no configuration has been used in this bundle since the start, I seem to remember that DependencyInjection folder was mandatory for a bundle at the beginning of SF2, just asking to learn.

[OskarStark]

One could extend the class as it is not final

[greg0ire]

Yeah that's highly improbable though… but let's not take any chances, we don't know what crazy things people are up to.

[franmomu]

Makes sense, I thought the same that no one would extend those classes, but you are right, better safe than sorry.