Remove unnecessary logback logger definition

Remove unnecessary logback logger definition

org.simantics logger is basically identical with the root logger what
comes to configuration. This specific logger can be removed to keep the
configuration clear and simple.

Also, this makes it more complicated to programmatically modify loggers
as both the root logger and org.simantics logger has to be
programmatically configured

gitlab #216

Change-Id: I8e27ea03ad3c5b6f2654edf0a7d863319e06bfa7