remove redundant assertion

Author: cmungall

linkml_model/model/schema/array.yaml

@@ -50,7 +50,6 @@ classes:
       - array_linearization_order
     slot_usage:
       elements:
-        required: true
         description: >-
           the collection of values that make up the array. The elements have a *direct* representation which is
           an ordered sequence of values. The elements also have an *array interpretation*, where each