Change required to false in sonata_type_model_list to avoid error "An invalid form control with name='' is not focusable"

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | sonata-project/SonataAdminBundle#1994, #257
| License       | MIT

My previous PR sonata-project/SonataAdminBundle#2455 didn't fix the issue properly (it introduced a problem with label). So I have [reverted the change](https://github.com/sonata-project/SonataAdminBundle/commit/45dc49f2f554871c643f84174ae509ec8e994341) and prepared this new PR to fix the issue.