Remove net.sf.okapi.steps.rainbowkit.postprocess.Parameters addMissingCodes option

It was revealed that a solution for issue #980 (closed) was improved in the scope of the net.sf.okapi.steps.rainbowkit.postprocess.Merger refactoring - the missed codes are not copied from source to target now. So, the net.sf.okapi.steps.rainbowkit.postprocess.Parameters addMissingCodes and all related code is redundant and can be removed.