Move spring-shell-samples to a separate repository

To keep the project codebase clean, I propose moving the samples to a separate repository — just like we do in other projects:

* https://github.com/spring-projects/spring-security-samples
* https://github.com/spring-projects/spring-amqp-samples
* https://github.com/spring-projects/spring-data-examples