Revert Yarn workspace changes from #6056 for now.

Yarn currently fails to add the correct symlinks in some cases. See https://github.com/yarnpkg/yarn/issues/4289

.yarnrc

@@ -1 +0,0 @@
-workspaces-experimental true

lerna.json

@@ -21,10 +21,5 @@
         "test/**"
       ]
     }
-  },
-  "packages": [
-    "packages/*"
-  ],
-  "npmClient": "yarn",
-  "useWorkspaces": true
+  }
 }

package.json

@@ -70,8 +70,5 @@
       "eslint --format=codeframe --fix --rulesdir='./eslint_rules'",
       "git add"
     ]
-  },
-  "workspaces": [
-    "packages/*"
-  ]
+  }
 }