[INTERNAL] removed prefix method from Util class

Advantage ? none ... well it shows jid == null as unknown which does not
help anyone ... especially when new developers start reading the
Saros log file. Again: a JID object should not be null.

Change-Id: I9d08c6fae5d84e3fccd432d51fa1b578e1ab1f29
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1320
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>