Make extruder_nr size_t in setExtruder_addPrime
It was size_t or unsigned int in all functions that call this function, so the if statement that checks if it's -1 will never occur. Then we can safely make this size_t here as well.
The extruder number should never be -1 after the settings system has had their go on the input obtained from the front-end.
Contributes to issue CURA-4410.