Remove ancient gitlab-flow page from navigation

Remove navigation item to global navigation

As part of Full revision of GitLab Flow document (gitlab!125799 - closed) and the blog post that @csaavedra1 anticipates publishing on July 27, let's take the ancient GitLab Flow (https://docs.gitlab.com/ee/topics/gitlab_flow.html) page out of the left nav as prep for taking the page down entirely.

Assigning @susantacker as the reviewer because she's aware of gitlab!125799 (closed) and needs no context for why this page will be removed.

Related

• Related to https://docs.google.com/document/d/192yXbsUQ1wPhcmWiWhysS-Z7Y-WAD3X98Fgv0_CNzJA
• Related to Gitlab Flow Docs - Adding Enablement Content fo... (gitlab#378297 - closed)
• Related to Remove links to GitLab Flow document (gitlab!127257 - merged)