remove deprecated corebundle

[garak]

Subject

Remove deprecated SonataCoreBundle

I am targeting 3.x, because there's no BC break.

Closes #423

Changelog

### Fixed
- Fix SonataCoreBundle deprecations

[core23]

Can you verify, that there is no usage for this bundle?

[garak]

Can you verify, that there is no usage for this bundle?

I did a search for related namespaces and ran tests.

[core23]

Keep in mind, that we use multiple namespaces:
https://github.com/sonata-project/SonataCoreBundle/blob/3.x/composer.json#L70-L73

[core23]

Just tried the online search...

SonataNotificationBundle/src/SonataNotificationBundle.php

Line 16 in 57b478a

use Sonata\CoreBundle\Form\FormHelper;

[garak]

You're right, I forgot to search for CoreBundle.
But now I'm not sure how to replace following classes:

• Sonata\CoreBundle\Form\FormHelper (can't find it in form-extensions)
• Sonata\CoreBundle\Exception\InvalidParameterException (not sure where to search it)

[core23]

Unfortunately we can't do this on the stable branch for now

[garak]

What is blocking?

[core23]

We have a complex dependency tree we'd like to solve: sonata-project/dev-kit#216

Our next step is the admin bundle, so we can release all other bundles that rely on it

[core23]

Closing this in favor of #440