Removed extra filter on integrator input

Since Connection objects have a default synapse of 0.005s, there's an easy mistake that can be made when defining the input and output passthrough Nodes inside a network.  We should always make sure that those input and output passthrough Nodes have ```synapse=None```, since otherwise they'll have an extra lowpass filter added on to their behaviour in addition to whatever the user specifies when connecting them up to something.