Build: Remove unused spawnback dependency

Follows-up c636c55, which removed use of this back in 2015.

Author: Krinkle

Gruntfile.js

@@ -1,5 +1,4 @@
-var spawnback = require( "spawnback" ),
-	jqueryContent = require( "grunt-jquery-content" );
+var jqueryContent = require( "grunt-jquery-content" );
 
 module.exports = function( grunt ) {
 

package-lock.json

@@ -1,28 +1,9 @@
 {
   "name": "learn.jquery.com",
-  "title": "jQuery Learning Site",
-  "description": "jQuery Foundation site for learning jQuery and JavaScript",
+  "private": true,
   "version": "0.6.29",
-  "homepage": "https://learn.jquery.com",
-  "author": {
-    "name": "jQuery Foundation and other contributors"
-  },
-  "repository": {
-    "type": "git",
-    "url": "git://github.com/jquery/learn.jquery.com.git"
-  },
-  "bugs": {
-    "url": "https://github.com/jquery/learn.jquery.com/issues"
-  },
-  "licenses": [
-    {
-      "type": "MIT",
-      "url": "https://github.com/jquery/api.jquery.com/blob/main/LICENSE.txt"
-    }
-  ],
   "dependencies": {
     "grunt": "1.5.3",
-    "grunt-jquery-content": "3.1.0",
-    "spawnback": "1.0.0"
+    "grunt-jquery-content": "3.1.0"
   }
 }