[ANE-2564] Remove fossa snippets subcommand

[spatten]

Overview

Delivers ANE-2565

Now that we have fossa analyze --snippet-scan in GA, we no longer need the fossa snippets subcommand.

Also, here's a Slack thread where Cortez notes that the existence of fossa snippets and fossa analyze --snippet-scan is causing confusion.

This PR removes the subcommand and any associated documentation.

Millhone is still used by Jars in Containers, so we will keep Millhone around for that.

Acceptance criteria

• The fossa snippets subcommand no longer exists
• Add documentation and references to the fossa snippets subcommand are removed

Testing plan

• The build passes
• You can no longer run fossa snippets
• fossa help does not mention fossa snippets
• Read through the docs and make sure there are no more references to fossa snippets

Risks

Metrics

References

Checklist

• I added tests for this PR's change (or explained in the PR description why tests don't make sense).
• If this PR introduced a user-visible change, I added documentation into docs/.
• If this PR added docs, I added links as appropriate to the user manual's ToC in docs/README.ms and gave consideration to how discoverable or not my documentation is.
• If this change is externally visible, I updated Changelog.md. If this PR did not mark a release, I added my changes into an ## Unreleased section at the top.
• If I made changes to .fossa.yml or fossa-deps.{json.yml}, I updated docs/references/files/*.schema.json AND I have updated example files used by fossa init command. You may also need to update these if you have added/removed new dependency type (e.g. pip) or analysis target type (e.g. poetry).
• If I made changes to a subcommand's options, I updated docs/references/subcommands/<subcommand>.md.