Drop references to planning docs that are not in the tree

The MachineKinematics comments pointed at docs/superpowers plan files,
which are gitignored working notes.

src/libslic3r/GCode/MachineKinematics.hpp

@@ -15,7 +15,7 @@ namespace Slic3r {
 //
 // This is a seam for writer-generated movement only.  Start/end/custom G-code,
 // classic wipe-tower output and GCodeWriter::extrude_arc_to_xy() do NOT pass
-// through it; see doc in 09-orca-machinekinematics-split-plan.md section 6.
+// through it: they write machine coordinates directly.
 class MachineKinematics
 {
 public:

tests/fff_print/test_gcodewriter.cpp

@@ -1038,7 +1038,7 @@ SCENARIO("Belt: start-gcode prepare-stage moves keep their real Z", "[GCode][bel
 
 // ---------------------------------------------------------------------------
 // Regression tests for the two latent bugs the MachineKinematics refactor
-// preserved deliberately (see 09b-latent-bug-fix-plan.md).
+// preserved deliberately and the follow-up commit fixed.
 // ---------------------------------------------------------------------------
 
 // Bug 1. _travel_to_z() emits full XYZ whenever the mapping must emit every