[examples] Remove old Spring example

### Description

https://github.com/ClickHouse/clickhouse-java/pull/3096 contains better spring service example so `example/demo-service` should be remove after the merge.

Inspect example/demo-service and review PR 3096 to confirm the newer Spring service example is available. Remove the obsolete example after that merge, then search for references to example/demo-service and run the relevant project checks to ensure nothing still depends on it.

Written by the indexing model from the issue text.