Unnecessary heating before cut

[RickSeiden]

When you select Cut Filament from the menu, it asks you what temperature to set. This makes sense if the filament is in the nozzle and the nozzle is not at temperature.

When the filament is not at the nozzle, however, it's completely unnecessary to bring the nozzle to temperature.

The system should check the filament sensor at the extruder and at the MMU selector to see if the filament needs to be retracted at all. If the filament sensor at the extruder does not indicate there is filament there, then no heating should occur.

[Ro3Deee]

I think this PR rezolves it:
prusa3d/Prusa-Firmware#4208