Remove the "representing GitLab on Twitter" link from the "Twitter response workflow" page.

This removes the broken and obsolete "representing GitLab on Twitter" link from https://about.gitlab.com/handbook/marketing/community-relations/community-advocacy/workflows/twitter/#workflow.

Note: The developer-relations directory was renamed community-marketing in commit 6f2e7a78 and the developer-advocacy page that contained the linked "Representing GitLab on Twitter" section was deleted in commit 9130ae7c.

@evhoffmann Should I put something else in place of the old link?