test: remove e2e test for dart sass

It doesn't add much value and interferes webpack 4 tests

Author: haoqunjiang

packages/@vue/cli-service/__tests__/serve.spec.js

@@ -168,17 +168,6 @@ test('serve with no public dir', async () => {
   )
 })
 
-test('dart sass', async () => {
-  const project = await create('test-dart-sass', exports.defaultPreset = {
-    useConfigFiles: false,
-    cssPreprocessor: 'dart-sass',
-    plugins: {}
-  })
-
-  // should build successfully
-  await project.run('vue-cli-service build')
-})
-
 test('use a single websocket connection for HMR', async () => {
   const project = await create('e2e-serve-hmr', defaultPreset)