Use mode from the loader context

See https://github.com/webpack/webpack/pull/9140.

examples/webpack/worker-loader.js

@@ -2,14 +2,7 @@ const build = require('../../tasks/serialize-workers').build;
 
 function loader() {
   const callback = this.async();
-
-  let minify = false;
-
-  // TODO: remove when https://github.com/webpack/webpack/issues/6496 is addressed
-  const compilation = this._compilation;
-  if (compilation) {
-    minify = compilation.compiler.options.mode === 'production';
-  }
+  const minify = this.mode === 'production';
 
   build(this.resource, {minify})
     .then(chunk => {