refactor(@angular/cli): cleanup leftover tsconfig reading logic

Followup to angular#7937

Author: filipesilva

packages/@angular/cli/models/webpack-configs/common.ts

@@ -157,8 +157,6 @@ export function getCommonConfig(wco: WebpackConfigOptions) {
     extraPlugins.push(new NamedLazyChunksWebpackPlugin());
   }
 
-  // Read the tsconfig to determine if we should prefer ES2015 modules.
-
   return {
     resolve: {
       extensions: ['.ts', '.js'],

packages/@angular/cli/models/webpack-configs/production.ts

@@ -121,11 +121,8 @@ export function getProdConfig(wco: WebpackConfigOptions) {
     uglifyCompressOptions.passes = 3;
   }
 
-  // Read the tsconfig to determine if we should apply ES6 uglify.
-  const tsconfigPath = path.resolve(projectRoot, appConfig.root, appConfig.tsconfig);
-  const tsConfig = readTsconfig(tsconfigPath);
-  const supportES2015 = tsConfig.options.target !== ts.ScriptTarget.ES3
-    && tsConfig.options.target !== ts.ScriptTarget.ES5;
+  const supportES2015 = wco.tsConfig.options.target !== ts.ScriptTarget.ES3
+    && wco.tsConfig.options.target !== ts.ScriptTarget.ES5;
 
   if (supportES2015) {
     extraPlugins.push(new UglifyJSPlugin({