RelEx2Logic clean-up-inst implementation

Specified in http://wiki.opencog.org/w/RelEx2Logic#Cleaning_Up_Instances

I was basically following the recommandation

> About this issue, we talked about cleaning the instance nodes that without definite-FLAG constraint, but please note that that constraint is only available for nouns, which are related with ConceptNode. I guess for all the adjective/adverb-extended ConceptNode, we can clean all the related instance nodes for now. For the PredicateNode, I think we should keep the instance, as they will be evaluated.

Are there other word instances that should be cleaned?