Removing extensions references

[emchap]

Basing this on https://docs.google.com/document/d/1nEao2QS5O-AWMvi2i0E8YH2GqgSibRdeDjM5oNa7udE/edit and https://about.sourcegraph.com/blog/integrations-update .

Since 2022-08-26 will be my last day, I'm assigning @hopkinscl to see this over the finish line. Main outstanding changes:

• 101's LMS content needs an updated set of screenshots + descriptions for the new flow to export a CSV (this will also need to be passed on to https://github.com/sourcegraph/accounts/issues/6716 as part of our collaboration with them)
• 102's LMS content needs an updated conclusion of Unit 3, and updated flow for Unit 4 (reflecting new flow for enabling open-in-editor if needed, and for configuring it)
• 102's quiz questions in unit 4 need a new question reflective of the new process to enable search export

[emchap]

@ryankscott More context in the opening PR comment, but I think you'll probably ultimately be the reviewer here (just not yet).