Remove "Useful Links" section from upgrade guide

- Now that the upgrade guide is part of the v3 docs, these links aren't
needed

Authored-by: Greg Cobb <gcobb@pivotal.io>

Author: Gerg

docs/v3/source/includes/upgrade_guide/upgrade_guide.md

@@ -542,8 +542,3 @@ Tasks are one-off jobs that are intended to execute a droplet, stop, and be clea
 Examples of this include database migrations and running batch jobs.
 
 Read more about the [task resource](#tasks).
-
-## Useful Links
-
-1. [V3 API Proposals](https://docs.google.com/document/d/1g48YPBfwXT8kNrJYroBNkhlCtROe53INEEzvhQaAUOI)
-2. [CAPI (Cloud Controller API) Slack Channel](https://cloudfoundry.slack.com/messages/C07C04W4Q)