[Draft] Eliminate required GUI for importDXF

[chennes]

Bug reported by forums member tarrox -- the importDXF code assumes the existence of the GUI, but does not actually need it (a FreeCAD.GuiUp check suffices to protect the one line of code that was using the view provider).

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• Discussed in forum: [BUG] Error using importDXF without GUI
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[hyarion]

There is a variable gui = FreeCAD.GuiUp in this file that could be used instead but that doesn't seem to be consistently used.
I wouldn't mind if it was cleaned up and only one way was used, but either way I think this PR is ok to be merged.

[yorikvanhavre]

this gui = FreeCAD.GuiUp is probably a leftover from some earlier structure... In any case indeed this should totally work without the GUI. Thanks for the fix!