Add route to remove nodes for a chain on demand.

[nsjames]

It will be common for nodes and chains to go down and they should be remove from the reference.

Right now the front-end has to traverse the array of nodes to find an available network each time.
The front end can notify the backend of fallen nodes but it might get notified multiple times by many users at once.

https://github.com/EOSPortal/eosportal-front/blob/master/src/views/Chain.vue#L88