[MIE-1434] add the remove activityItem function

After unfollow a member, the previous activity when start following that member, persist. I resolved it by adding the removeActivity function
Right, that is what I was getting at. I think we may need to do something like this. Keep in mind that we do not publish a verb for DELETEs because by default Neo4j doesn't return a verb when the cypher is a "DELETE", however we can grab the verb from the url parameters.