Update BDOrthoIGN.geojson to exclude Channel Islands

[osmuser63783]

This is my first time contributing here, I hope what I am doing is right.

The issue I am trying to fix is that the iD editor defaults to the BDOrtho IGN layer when editing in the Channel Islands, but this layer doesn't have any data in the Channel Islands, only in France. This has been reported before in #1260 and #2101.

I opened https://github.com/osmlab/editor-layer-index/blob/gh-pages/sources/europe/fr/BDOrthoIGN.geojson in QGIS and subtracted the Channel Islands admin boundaries, which I got from OSM. The result is a polygon with 'holes' for the Channel Islands: https://github.com/osmuser63783/editor-layer-index/blob/db397a513e3fdf600ac92f47f4c8c88d6a4ab8e3/sources/europe/fr/BDOrthoIGN.geojson

I haven't tested this on a test server with iD or anything like that. Please let me know if this should fix it!

[Intrinsica1]

This adds a lot of unnecessarily precise geometry to roughly drawn bounds. I think the best thing here is to simply retrace the existing geometry. Or ideally obtain the actual bounds from IGN!

I have already started retracing a bit.

Decided to look into getting bounds directly instead.

I've found ORTHOIMAGERY.ORTHOPHOTOS.GRAPHE-MOSAIQUAGE:graphe_bdortho at https://data.geopf.fr/wfs/ows?SERVICE=WFS&VERSION=2.0.0&REQUEST=GetCapabilities

The issue is that it combines good bounds, with overly-complex internal geometry:

Dissolving it has been problematic.

[arch0345]

I've simplified the geometry, inner rings aren't really needed to exclude these islands

[osmuser63783]

Thanks a lot!