remove obsolete mongo demo
remove obsolete mongo demo

This code is bitrotten, and does more harm than good to those stumbling
upon it while trying to find an example of how to use persistent-mongoDB.

erikd/persistent,gbwey/persistent,psibi/persistent,plow-technologies/persistent,yesodweb/persistent,gbwey/persistent,paul-rouse/persistent,bitemyapp/persistent,gbwey/persistent,naushadh/persistent,creichert/persistent