DXF export: leave out the centerline option and the 3ds Max preset for now

The VPORT view direction fix and its test stay. The Export Strokes as
Centerlines option and the preset button are not in the documented DXF/DWG
export options, so they wait for the owner's decision.

docs/mcp.md

@@ -2277,14 +2277,6 @@ selected objects in their layers (every export takes it), and `useArtboards: tru
 artboard holding the art over it. What DXF can't hold (gradients and patterns as one colour, blending, opacity masks,
 raster effects, clipping) comes back in `warnings`.
 
-`centerlineStrokes: true` writes each stroked path once, as a zero-width centerline (its fill and stroke outline
-are left out): the linework 3D and CAM software extrude or sweep along, instead of the outline of the stroke's
-width. For linework intended for 3ds Max, Export As → DXF has a **3ds Max — R12 Linework** preset: millimetres, R12
-polylines (curves are flattened), editable strokes as centerlines, 256 colours and outlined text, the version older
-CAD importers read most reliably. Review the export warnings: R12 drops images and fill interiors. Through
-`document.exportDxf` the preset is `version: "R12"`, `unit: "mm"`, `preserve: "editability"`,
-`centerlineStrokes: true`, `colors: 256` and `outlineText: true`.
-
 DWG can't be written (no openly licensed writer exists): `document.formats` lists it under `unsupported` with that
 hint, and exporting to it answers with the hint to export DXF. PICT files are not supported either and say so when
 opened.