Use union type instead of enum for feature format

src/ol/format/MVT.js

@@ -4,7 +4,6 @@
 //FIXME Implement projection handling
 
 import FeatureFormat, {transformGeometryWithOptions} from './Feature.js';
-import FormatType from './FormatType.js';
 import GeometryLayout from '../geom/GeometryLayout.js';
 import LineString from '../geom/LineString.js';
 import MultiLineString from '../geom/MultiLineString.js';
@@ -238,10 +237,10 @@ class MVT extends FeatureFormat {
   }
 
   /**
-   * @return {import("./FormatType.js").default} Format.
+   * @return {import("./Feature.js").Type} Format.
    */
   getType() {
-    return FormatType.ARRAY_BUFFER;
+    return 'arraybuffer';
   }
 
   /**