[MPMD-169] Support multi-threaded mode of PMD 5

PMD 5 supports executing in multi-threaded mode, which should be a performance improvement in case you have multiple cores.

This patch removes the workaround introduced while adding PMD 5 support and simply let PMD decided, whether to switch
on multi-threaded mode. PMD uses `Runtime.availableProcessors()` to determine the thread-pool size.

There is no additional test case added. However, all tests including integration tests work
(e.g. `mvn clean integration-test -Prun-its` is successful for me).

Jira ticket: https://jira.codehaus.org/browse/MPMD-169