Belt: drop the per-extrusion transform determinant

The mesh transform is a rotation and an axis permutation, so its
determinant is always 1; rebuilding the forward transform on every
extrusion to divide the flow by it changed nothing.

Raised in Hanif Koh's review of #14394.

src/libslic3r/GCode.cpp

@@ -8402,20 +8402,6 @@ std::string GCode::_extrude(const ExtrusionPath &path, std::string description,
     auto _mm3_per_mm = path.mm3_per_mm * this->config().print_flow_ratio;
     _mm3_per_mm *= filament_flow_ratio;
 
-    // Belt printer: compensate for any volume change introduced by the mesh
-    // forward transform.  path.mm3_per_mm is derived from slicer-frame layer
-    // height × line width, but a slicer-frame slab of volume V maps under the
-    // back-transform to a machine-frame region of volume V / |det(T)|.  The
-    // mesh transform is now rotation ∘ pre-remap, both orthogonal, so |det(T)|
-    // is always 1 and this is currently a no-op; it is retained as a guard in
-    // case a non-orthogonal mesh transform is ever reintroduced.  (Machine-frame
-    // shear/scale acts on the g-code in BeltKinematics, not here.)
-    if (m_config.belt_printer.value) {
-        double det = std::abs(BeltTransformPipeline::build_forward_transform(m_config).linear().determinant());
-        if (det > EPSILON)
-            _mm3_per_mm /= det;
-    }
-
     if (path.role() == erTopSolidInfill) {
         _mm3_per_mm *= m_config.top_solid_infill_flow_ratio;
     } else if (path.role() == erBottomSurface) {