remove select required attribute

[apapacy]

I am targeting this branch, because it is patch.

Closes #4022, #4486

Changelog

### Removed
Attribute 'required' from HTML element select in Resources/views/Form/Type/sonata_type_model_autocomplete.html.twig

### Fixed
Error on save form with autocomplete form field ('sonata_type_model_autocomplete') where required is set to 'true'

### Security
Input field in 'sonata_type_model_autocomplete' already  have attribute 'required'

Subject

Fix problem with required autocomplete field on save from.

[greg0ire]

Please associate your meta.ua email address with your Github account, or change the email in your commits to an address already associated with it.

[greg0ire]

This select does not have a name attribute. I think the right fix might be to specify one.

[apapacy]

The name attribute present in input type='hidden' element.
https://github.com/sonata-project/SonataAdminBundle/blob/3.x/Resources/views/Form/Type/sonata_type_model_autocomplete.html.twig#L32

If you define name attribute for select element - then this value will be sent to the server that is undesirable.

[greg0ire]

Isn't select2 supposed to remove it just before submit? Not sure why we are building the hidden fields ourselves either.

[greg0ire]

I'm not sure they are even supposed to be there anymore : https://select2.github.io/announcements-4.0.html#hidden-input

[apapacy]

Yes, the hidden input element must be deleted! In this regard, Im close this pull request.