logback: revert commit 1cc57b1efaced76d22e5b561f861c6daf981f249

Not all layout tags had to be replaced by encoder tags, which now even caused problems.
This patch reverts this situation. The review that was acked before was this:
https://rb.dcache.org/r/7554

packages/system-test/src/main/skel/etc/logback.xml was deleted by a later patch after, which
makes it unnessecary to revert this files's changes.

Acked-by: Gerd
Target: master
Require-book: no
Require-notes: no
Patch: http://rb.dcache.org/r/7570/