Stop locking display override field

Problem/Motivation

During development, it was decided to lock display override field storage, so users can't change cardinality (locked to unlimited) or translation strategy (manage elsewhere).

However, 2 changes happened since then:

• #3544026: Cardinality constraints for slots: No need to constraint the cardinality to unlimited anymore, the builder is handling any value.
• #3548884: SourceValueItem field synchronized translations: Builders have now a setting they want to control and not available anywhere else: Symmetric/Asymmetric translations. Let's check if the switch is correctly handled by #3555110: Symmetric translation

Proposed resolution

Remove this logic from EntityViewDisplayFormTrait

Low priority, let's address this after 1.0.0. I have just open the ticket to host the current work, which was helpful to review #3611826: Display Builder override saves to wrong language on multilingual sites.