Don't define functions in the prototype

If `VOID` is used, TypeScript is not able to figure out what the function parameters are.

Before:
```
$ npx tsc | wc -l
    1188
```

After:
```
$ npx tsc | wc -l
    1169
```

src/ol/render/Feature.js

@@ -1,7 +1,6 @@
 /**
  * @module ol/render/Feature
  */
-import {VOID} from '../functions.js';
 import {extend} from '../array.js';
 import {createOrUpdateFromCoordinate, createOrUpdateFromFlatCoordinates, getCenter, getHeight} from '../extent.js';
 import GeometryType from '../geom/GeometryType.js';
@@ -208,6 +207,13 @@ class RenderFeature {
     return 2;
   }
 
+  /**
+   * @return {undefined}
+   */
+  getStyleFunction() {
+    return undefined;
+  }
+
   /**
   * Get the type of this feature's geometry.
   * @return {import("../geom/GeometryType.js").default} Geometry type.
@@ -263,10 +269,4 @@ RenderFeature.prototype.getSimplifiedGeometry =
     RenderFeature.prototype.getGeometry;
 
 
-/**
- * @return {undefined}
- */
-RenderFeature.prototype.getStyleFunction = VOID;
-
-
 export default RenderFeature;