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).

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) {