PROBABLY-TO-UNDO: use maven-plugin-testing-harness

Seems not to be worth the trouble of the additional enforcer configuration.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
FAILED-TRY: use maven-plugin-testing-harness

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>