dxf 12 export includes clips and masks and their clipped and masked objects

Split out from discussion in inbox#3910 (moved)

Steps to reproduce:

• open clips_and_masks.svg in Inkscape.
  • File includes an ellipse clipped by another ellipse, and an unused clip
  • Also a star masked by another star, and an unused mask (in the shape of an M)
• export to dxf 12

What happened?

• all clips, including unused ones are included in the export
• clipped objects are included too

What should have happened?

• just the clipped objects should be included (or maybe their clips too, but that doesn't occur in dxf 14 export

Sample attachments:

• test file from original issue
• ununsed_clip.svg original test case, only includes clip info
• clips_and_masks.svg file from this issue, see steps to replicate

clips_and_masks.svg export to dxf:

dxf 12	dxf 14	dxf 12 in 0.92.5

Version Info:

• Inkscape 1.1-dev (d1a926bd5e, 2020-11-14) with extensions at head and master (27ddbda2) Linux Mint 20
• Inkscape 1.0.1 (1867368bb3, 2020-10-26) with extensions at head Linux Mint 20