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;

src/ol/render/canvas/Replay.js

@@ -2,7 +2,6 @@
  * @module ol/render/canvas/Replay
  */
 import {getUid} from '../../util.js';
-import {VOID} from '../../functions.js';
 import {equals, reverseSubArray} from '../../array.js';
 import {asColorLike} from '../../colorlike.js';
 import {buffer, clone, coordinateRelationship, createEmpty, createOrUpdate,
@@ -456,6 +455,11 @@ class CanvasReplay extends VectorContext {
     this.hitDetectionInstructions.push(this.beginGeometryInstruction2_);
   }
 
+  /**
+   * FIXME empty description for jsdoc
+   */
+  finish() {}
+
   /**
    * @private
    * @param {CanvasRenderingContext2D} context Context.
@@ -1081,10 +1085,4 @@ class CanvasReplay extends VectorContext {
 }
 
 
-/**
- * FIXME empty description for jsdoc
- */
-CanvasReplay.prototype.finish = VOID;
-
-
 export default CanvasReplay;