Remove Camel-K extension

[manusa]

Description

The Camel-K extension was deprecated in #5107 due to the publishing of an official extension (apache/camel-k#3994).

The Camel-K project now provides an official extension based on our Java Generator: https://github.com/apache/camel-k/blob/main/docs/modules/ROOT/pages/apis/java.adoc

The next step is to completely remove the extension from our code-base.

In place of the extension entry we should add a document to point to the location of the officially maintained extension in the Camel K repo (also in the main README.md).