build: remove local yarn (#19981)

We use the globally installed yarn now. The local yarn was used in
`check-environment.js` only, which results in the `--integrity` check
always failing (if dependencies were installed with the global yarn).

package.json

@@ -105,7 +105,6 @@
     "vrsource-tslint-rules": "5.1.1",
     "webpack": "1.12.9",
     "xhr2": "0.1.4",
-    "yargs": "9.0.1",
-    "yarn": "1.0.2"
+    "yargs": "9.0.1"
   }
 }

tools/npm/check-node-modules.js

@@ -17,8 +17,7 @@ var PROJECT_ROOT = path.join(__dirname, '../../');
 // tslint:disable:no-console
 function checkNodeModules(logOutput, purgeIfStale) {
   var yarnCheck = childProcess.spawnSync(
-      './node_modules/.bin/yarn check --integrity',
-      {shell: true, cwd: path.resolve(__dirname, '../..')});
+      'yarn check --integrity', {shell: true, cwd: path.resolve(__dirname, '../..')});
 
   var nodeModulesOK = yarnCheck.status === 0;
   if (nodeModulesOK) {

yarn.lock

@@ -7887,10 +7887,6 @@ yargs@~3.10.0:
     decamelize "^1.0.0"
     window-size "0.1.0"
 
-yarn@1.0.2:
-  version "1.0.2"
-  resolved "https://registry.yarnpkg.com/yarn/-/yarn-1.0.2.tgz#d1b8f4b6d3b0684e86f63a072ac630995b8b7b0a"
-
 yeast@0.1.2:
   version "0.1.2"
   resolved "https://registry.yarnpkg.com/yeast/-/yeast-0.1.2.tgz#008e06d8094320c372dbc2f8ed76a0ca6c8ac419"