Added custom labels and possibility to remove button sonata_type form types

https://github.com/sonata-project/SonataDoctrineORMAdminBundle/pull/231

The existing "delete" buttons had a 'link_cancel' label which doesn't seem to exist anywhere.
Also, there's a bug fix regarding permissions validation for "delete" action that's also fixed here.

**DEPENDS ON sonata-admin > 2.2.3 but there is no release tag yet**