dcache: Remove rogue logback-test.xml file

Commit 6a3276cfdcd17c73a4b9d2e57cd7913b6d2b86cf added a custom logback-test.xml
file needed for a single test. The logback configuration however applied to
all tests in the dcache module and resulted in severily bloated log output during
testing.

Since then, the test has been modified and it doesn't appear to rely on the
custom logback configuration anymore. However, the logback-test.xml file
remained.

This patch removes the file.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5594/