Remove unsupported keyword OAS 3.0.x typing - additionalItems (#1609)

Co-authored-by: Jeremy Fiel <[email protected]>

.changeset/hungry-files-scream.md

@@ -0,0 +1,6 @@
+---
+"@redocly/openapi-core": patch
+"@redocly/cli": patch
+---
+
+Removed `additionalItems` from OAS 3.0.x typings. This keyword is not supported by the specification.

packages/core/src/types/oas3.ts

@@ -346,13 +346,6 @@ const Schema: NodeType = {
         return 'Schema';
       }
     },
-    additionalItems: (value: any) => {
-      if (typeof value === 'boolean') {
-        return { type: 'boolean' };
-      } else {
-        return 'Schema';
-      }
-    },
     additionalProperties: (value: any) => {
       if (typeof value === 'boolean') {
         return { type: 'boolean' };