chore(manager): remove unused jigsaw link

[ohnorobo]

This link is no longer used in

outline-apps/server_manager/messages/en.json

Line 2 in d2d72a1

"about-outline": "Outline is an open source project created by the Outline Foundation to provide a safer way for news organizations and journalists to access the internet.<br><br> Outline is powered by <a href={shadowsocksUrl}>Shadowsocks</a> and is still an early stage product. You can contribute to the code on <a href={gitHubUrl}>GitHub</a>, and follow us on <a href={redditUrl}>Reddit</a> and <a href={mediumUrl}>Medium</a> to hear when we expand to more platforms and add new features.",

/ other language files

It was deleted in #2675