fix: Drop @types/webpack dependency

Author: jantimon

package.json

@@ -32,7 +32,6 @@
     "@types/loader-utils": "1.1.3",
     "@types/node": "10.11.4",
     "@types/tapable": "1.0.4",
-    "@types/webpack": "^4.0.0",
     "appcache-webpack-plugin": "^1.4.0",
     "commitizen": "3.0.2",
     "css-loader": "^1.0.0",

typings.d.ts

@@ -1,10 +1,9 @@
-import { Plugin } from "webpack";
 import { AsyncSeriesWaterfallHook } from "tapable";
 import { Options as HtmlMinifierOptions } from "html-minifier";
 
 export = HtmlWebpackPlugin;
 
-declare class HtmlWebpackPlugin extends Plugin {
+declare class HtmlWebpackPlugin {
   constructor(options?: HtmlWebpackPlugin.Options);
 }