remove docs/code related to generate-pipeline command

- generate-pipeline command was used to generate tekton pipeline in v1 and it never got implemented in v2
- #8431 removed some code related to that, we've been in v2 for over a year and v1 is no longer under maintained
- No issues about generate-pipeline is broken, it is safe there are no users for that command
- We should clean up genreate-pipeline references to avoid confusion, that is to say-- we should remove the command entirely

Some places that references generate-pipeline command :
- https://github.com/GoogleContainerTools/skaffold/blob/085f74c5acd75c51f75715c0bb082ed9f18bddf2/docs-v2/data/maturity.json#L330-L335
- https://github.com/GoogleContainerTools/skaffold/blob/88e1c1734ea0f1bd79a0f24783f9565057a0e46a/cmd/skaffold/app/cmd/generate_pipeline.go#L35-L43
- https://github.com/GoogleContainerTools/skaffold/blob/2ba7d48a2bece920ed8eabc801ae2b47dd9f0333/cmd/skaffold/app/cmd/cmd.go#L199