PFW-1458 Reduce loading test length

The issue is very apparent with a cold extruder. For example when starting a single color print with the MMU. I can hear grinding in the extruder gears at the very end of the sequence. This means the sequence is too long and we are
likely pushing the filament too far into the extruder.

The purpose of the test is to check if the filament is actually
present in the PTFE tube. My proposal is we reduce the length
of the test to only cover the *length* of the PTFE tube.
This will also reduce printing time a little bit.

Firmware/mmu2.cpp

@@ -318,9 +318,9 @@ bool MMU2::VerifyFilamentEnteredPTFE()
     uint8_t fsensorState = 0;
     // MMU has finished its load, push the filament further by some defined constant length
     // If the filament sensor reads 0 at any moment, then report FAILURE
-    current_position[E_AXIS] += MMU2_EXTRUDER_PTFE_LENGTH + MMU2_EXTRUDER_HEATBREAK_LENGTH - logic.ExtraLoadDistance();
+    current_position[E_AXIS] += MMU2_EXTRUDER_PTFE_LENGTH - logic.ExtraLoadDistance();
     plan_buffer_line_curposXYZE(MMU2_LOAD_TO_NOZZLE_FEED_RATE);
-    current_position[E_AXIS] -= (MMU2_EXTRUDER_PTFE_LENGTH + MMU2_EXTRUDER_HEATBREAK_LENGTH - logic.ExtraLoadDistance());
+    current_position[E_AXIS] -= (MMU2_EXTRUDER_PTFE_LENGTH - logic.ExtraLoadDistance());
     plan_buffer_line_curposXYZE(MMU2_LOAD_TO_NOZZLE_FEED_RATE);
 
     while(blocks_queued())