Remove untested, unCI droolsjbpm-integration.git/kie-maven-plugin-example

At the time of writing, droolsjbpm-integration.git/kie-maven-plugin-example is not part of the reactor build: https://github.com/kiegroup/droolsjbpm-integration/blob/31b8231fd32a256ccdae6aa5fc169f3aa30aed8e/pom.xml#L55-L57 which is expected. However, there is no CI/Jenkins job to test this module separately.

As a result, this module at the time of writing is dead code.

We got report from community user which "found" this example and tried to use it, only to discover this is broken.

Moreover, this example is not analogous to any of the archetype, because:
1. it defines a parent in the pom, and none of the archetype's result or expected examples require the user to define the parent pom for the user's KJAR project.
2. it defines tests which create the KieContainer in memory, which is non a sensible user test in a KJAR, where usually the KieContainer can be instantiated from classpath.

Conclusion: this module will be removed, since it's not active, and moreover is misleading for community users.
For good examples of basic KJAR usages, the Users SHALL be invited to be using the Kie Archetypes.

At the time of writing, there is no mention of the `kie-maven-plugin-example` in the documentation (https://docs.jboss.org/drools/release/7.53.0.Final/drools-docs/html_single/index.html) either.