Remove the breakingChanges member from the required trait (#2748)

The validation logic for required is done by the `ChangedNullability`
validator. Using breaking changes without context can be misleading,
e.g., adding `@required` is OK as long as it's accompanied by
`@clientOptional`.

Author: sugmanue

smithy-model/src/main/resources/software/amazon/smithy/model/loader/prelude.smithy

@@ -767,13 +767,6 @@ string pattern
 /// be present.
 @trait(
     selector: "structure > member"
-    breakingChanges: [
-        {
-            change: "add"
-            severity: "WARNING"
-            message: "If any consumers were previously omitting this member in operation inputs, making it required is backwards incompatible"
-        }
-    ]
 )
 structure required {}