Build: Make cdn_origin setting optional

Follows-up 6b39592. Not needed anymore. Keep the override-ability
for now, but make it no longer required so that we can use the same
three settings (url, username, password) for all WordPress-built
documentation sites.

Author: Krinkle

Gruntfile.js

@@ -323,7 +323,7 @@ grunt.registerTask( "build-index", function() {
 
 	function cdnSriLink( file, label ) {
 		var sri = "sha256-" + sriHashes[ `@cdn/${ file }` ].hashes.sha256,
-			cdnOrigin = grunt.config( "wordpress" ).cdn_origin;
+			cdnOrigin = grunt.config( "wordpress" ).cdn_origin || "https://code.jquery.com";
 		return `<a
 			class='open-sri-modal'
 			href='${ cdnOrigin }/${ file }'

config-sample.json

@@ -1,6 +1,5 @@
 {
 	"url": "vagrant.codeorigin.jquery.com",
 	"username": "admin",
-	"password": "secret",
-	"cdn_origin": "https://code.jquery.com"
+	"password": "secret"
 }