(See #122713 for context.)

go_router and go_router_builder examples have several external dependencies:

• adaptive_dialog
• provider

which violates https://github.com/flutter/flutter/wiki/Contributing-to-Plugins-and-Packages#dependencies

If these aren't critical to being able to provide a good example (I haven't looked at the details, but presumably they aren't), then we should remove them to reduce the repo's exposure.

(I've filed a single issue to group it, but they could certainly be removed individually, or only one removed if one is critical and the other isn't.)