Could we remove the resistor in parallel with voltage source or in series with current source?
As @venny mentioned, and the text they are the same with respect to the terminals a and b

For any Rl that you attach to node A and node B, you will get the same voltage or current because these are ideal voltage and current sources. 

If we look at 4.39a, they are the same because the voltage on Rp will always be equal to Vs. So if we we remove resistor Rp, it has no effect because the voltage at that node will always be equal to Vs. Make sense ?

The same is true for the current source, beacuse the current source will always provide that current.