src extensions includes deprecation message that links to nonexistent url

[AlexB3939]

When a user runs src ext or src extensions the following is displayed as part of the response:

DEPRECATED: We're in the process of removing Sourcegraph extensions with our September release.
Learn more: https://docs.sourcegraph.com/extensions/deprecation

However, the provided link leads to a page not found result:

[mrnugget]

I think we can remove the commands altogether now. The extensions have been deprecated ~1 year ago and removed ~7 months ago:

https://sourcegraph.sourcegraph.com/github.com/sourcegraph/sourcegraph/-/commit/80024726b7933e628120eef7f272740aadb074d0