deprecate: remove snap sm plugin as it has been moved to its own repo

[reubenmiller]

Proposed changes

Remove the tedge-snap-plugin example as it has been moved to its own repository which includes full workflows for testing and publishing to the Cloudsmith community repository.

The tedge-snap-plugin can now be found here, https://github.com/thin-edge/tedge-snap-plugin

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New example (non-breaking change which adds functionality)
• [ ] Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• [x] Documentation Update (if none of the other choices apply)

Checklist

• [x] I have read the CONTRIBUTING doc
• [x] I have signed the CLA (in all commits with git commit -s)
• [ ] I have added necessary documentation (if appropriate)

Further comments