docs: Remove blog post reference for jq in icinga2-api.md

[theFeu]

Remove a reference to a blog post about jq filter capabilities, due to the post being depublished

[oxzi]

Thanks for creating this PR.

Outsourcing documentation to a blog was never a good idea. Grepping for netways.de URLs showed multiple dead links.

$ git grep 'http.*netways.de' doc
doc/07-agent-based-monitoring.md:A more advanced example with better arguments is shown in [this blogpost](https://www.netways.de/blog/2016/03/21/check_by_ssh-mit-icinga-2/).
doc/09-object-types.md:HTTP proxy (Nginx, etc.) in front of the Elasticsearch instance. Check [this blogpost](https://blog.netways.de/2017/09/14/secure-elasticsearch-and-kibana-with-an-nginx-http-proxy/)
doc/12-icinga2-api.md:jq also has additional filter capabilities, as shown in [this blogpost](https://www.netways.de/blog/2018/08/24/json-in-bequem/).
doc/15-troubleshooting.md:More details in [this blogpost](https://www.netways.de/blog/2019/01/24/windows-blocking-icinga-2-with-ephemeral-port-range/)
doc/19-technical-concepts.md:* [The Icinga Config Compiler: An Overview](https://www.netways.de/blog/2018/07/12/the-icinga-config-compiler-an-overview/)
doc/19-technical-concepts.md:More details on this topic can be found in [this blogpost](https://www.netways.de/blog/2019/04/04/modern-c-programming-coroutines-with-boost/).

From this list, the followings are dead:

• https://www.netways.de/blog/2018/08/24/json-in-bequem/ (this PR)
• https://www.netways.de/blog/2019/01/24/windows-blocking-icinga-2-with-ephemeral-port-range/
• https://www.netways.de/blog/2018/07/12/the-icinga-config-compiler-an-overview/

@theFeu: Would you mind removing those as well? Thanks :)