Load to Nozzle allows potentially harmful purge when using Mk3s/MMU2 files

[mmcglumphy]

1. Using any PLA filament, Load to Nozzle.
2. Print a gcode file that was generated in Slic3r 2 using the single-nozzle Mk3s/MMU2s profiles.
3. Watch for the purge that happens immediately after bed leveling.

Since the filament is loaded all the way to the nozzle, the initial purge is excessive and results in a large blob being rapidly forced out at near bed level, followed by extruder grinding once the purge can no longer force filament fast enough to overcome the blob.

I'm not sure if this action is originating from the firmware or is a Slicer action, but the purge should not be allowed to occur if the extruder is currently in Load to Nozzle position at that height. This action feels harmful. Also after laying the long bead across the length of the bed, the return motion again runs into the blob.

(While learning the ins-and-outs of the MMU2, I have experienced this at least 3 times before realizing why I it was happening. It is very easy to forgot and leave the filament at the nozzle when printing older gcode files for the MK3 and then switching back to newer files.)

[mmcglumphy]

Looks like this is probably a dupe of #2069