Remove use-nodes handling in simfactory

Simfactory currently tries to determine which nodes the queuing system provides to a run. This list of nodes is not used anywhere, and the mechanism is specific to (probably) two flavours of PBS.

This mechanism is not necessary, since this functionality is contained in the individual run scripts. The run scripts also save the list of nodes into a file for possible later use.

The enclosed patch removes this functionality, simplifying the code.