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/playbooks/lint/pre.yaml b/playbooks/lint/pre.yaml
index 13ccb60..964416f 100644
--- a/playbooks/lint/pre.yaml
+++ b/playbooks/lint/pre.yaml
@@ -1,4 +1,3 @@
 - hosts: all
   roles:
-    - role: ensure-go
-      go_version: 1.16
+    - role: ensure-genmanifest