N°6414 Move existing AbstractSimpleField::Validate impl to custom validators

- LinkedSetField
- SelectObjectField
- MultipleChoicesField (warning this hierarchy contains non multiple value fields like SelectField !)

Also change AbstractValidator::Validate signature : now we are returning an array of error messages, so that we can return multiple ones

core/attributedef.class.inc.php

@@ -2477,8 +2477,6 @@ class AttributeLinkedSet extends AttributeDefinition
 			$oFormField->SetLnkAttributesToDisplay($aLnkAttributesToDisplay);
 		}
 
-		$oFormField->AddValidator(new LinkedSetValidator());
-
 		parent::MakeFormField($oObject, $oFormField);
 
 		return $oFormField;