Revert "Add support for new yarn workspaces config format (facebook#4001)"

This reverts commit 9690bc8.

Author: Timer

packages/react-dev-utils/workspaceUtils.js

@@ -32,8 +32,7 @@ const findPkgs = (rootPath, globPatterns) => {
 const findMonorepo = appDir => {
   const monoPkgPath = findPkg.sync(path.resolve(appDir, '..'));
   const monoPkg = monoPkgPath && require(monoPkgPath);
-  const workspaces = monoPkg && monoPkg.workspaces;
-  const patterns = (workspaces && workspaces.packages) || workspaces;
+  const patterns = monoPkg && monoPkg.workspaces;
   const isYarnWs = Boolean(patterns);
   const allPkgs = patterns && findPkgs(path.dirname(monoPkgPath), patterns);
   const isIncluded = dir => allPkgs && allPkgs.indexOf(dir) !== -1;