Switch to external genmanifest tool

Until now, we were using a Go program included in this repo to generate
the manifest. This program is being used in other projects, so it got
moved to the gomodules/webext repo.

This change removes the local Go files and switches to the external
genmanifest tool.

Change-Id: Id6168282122ae53ecb9e20a296d2f223e66b9f25
diff --git a/webpack.config.js b/webpack.config.js
index 5b897dc..2c94b12 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -73,7 +73,7 @@
       new WebpackShellPluginNext({
         onBuildEnd: {
           scripts:
-              ['go run tools/generateManifest.go -dest=' +
+              ['genmanifest -template templates/manifest.gjson -dest ' +
                path.join(outputPath, 'manifest.json') + ' ' +
                env.browser_target]
         }