Thanks for reporting, we are aware of this and it is a known in Apache Karaf 4.4.6, see SLF4J(W) console output when logging into Karaf console · Issue #4283 · openhab/openhab-core · GitHub. Once Karaf 4.4.7 is released, we can upgrade to it and this should be fixed then.
Until then I guess you can ignore it.
