Use union type instead of enum for feature format

src/ol/format/WKB.js

@@ -3,7 +3,6 @@
  */
 import Feature from '../Feature.js';
 import FeatureFormat, {transformGeometryWithOptions} from './Feature.js';
-import FormatType from './FormatType.js';
 import GeometryCollection from '../geom/GeometryCollection.js';
 import GeometryLayout from '../geom/GeometryLayout.js';
 import LineString from '../geom/LineString.js';
@@ -691,10 +690,10 @@ class WKB extends FeatureFormat {
   }
 
   /**
-   * @return {import("./FormatType.js").default} Format.
+   * @return {import("./Feature.js").Type} Format.
    */
   getType() {
-    return this.hex_ ? FormatType.TEXT : FormatType.ARRAY_BUFFER;
+    return this.hex_ ? 'text' : 'arraybuffer';
   }
 
   /**