FIX remove write from SearchableDropdownTrait

[johannesx75]

Description

SearchableDropdownTrait was calling write() on the underling DataObject.

Manual testing steps

1. Create Model with multiple has_one relations
2. Save DataObject in ModelAdmin

Issues

• 11284

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  • See picking the right version
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I don't have a full understanding on how and where SearchableDropdownTrait and the SearchableDropdown are used. So I'm not sure about test coverage for SearchableDropdownTrait.