Add inject example as a comment & rm the tests in rest-client codestart

Follow up on https://github.com/quarkusio/quarkus/pull/24544

Fixes #24553

Generally speaking, having a test on a remote rest endpoint is a bad practice. In this case, it is more testing the rest service and the rest-client implementation than the actual code. Mocking might be a better option but let's leave this choice open to the developer. I've added the injection code snippet as a comment directly in the service and removed the test.