When introduced we had no other way to set custom log levels than
to have a logback.xml specific to system-test. Nowadays log levels
can be adjusted through properties in our regular configuration
files. Hence, there is no need for the custom logback configuration.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7552/