Remove the bulk of the WebGL legacy code.

Things left to do:
* redo an icon layer example
* redo a clipping layer example
* update docs where WebGL renderers are mentioned

src/ol/layer/WebGLTile.js

@@ -1,43 +0,0 @@
-/**
- * @module ol/layer/WebGLTile
- */
-import BaseTileLayer from './BaseTile.js';
-import WebGLTileLayerRenderer from '../renderer/webgl/TileLayer.js';
-
-/**
- * @typedef {import("./BaseTile.js").Options} Options
- */
-
-
-/**
- * @classdesc
- * For layer sources that provide pre-rendered, tiled images in grids that are
- * organized by zoom levels for specific resolutions.
- * Note that any property set in the options is set as a {@link module:ol/Object~BaseObject}
- * property on the layer object; for example, setting `title: 'My Title'` in the
- * options means that `title` is observable, and has get/set accessors.
- *
- * @api
- */
-class WebGLTileLayer extends BaseTileLayer {
-
-  /**
-   * @param {Options=} opt_options Tile layer options.
-   */
-  constructor(opt_options) {
-    super(opt_options);
-  }
-
-  /**
-   * Create a renderer for this layer.
-   * @param {import("../renderer/webgl/Map.js").default} mapRenderer The map renderer.
-   * @return {import("../renderer/Layer.js").default} A layer renderer.
-   * @protected
-   */
-  createRenderer(mapRenderer) {
-    return new WebGLTileLayerRenderer(mapRenderer, this);
-  }
-
-}
-
-export default WebGLTileLayer;