- License: Remove "CC-BY-SA-2.0" as explicit license type, but make note about unclear provenance of some images

- Docs: Update license badges to reflect lack of (known) CC-BY-SA-2.0 usage
- Replace CC-BY-SA-2.0 polygon image (thanks, @mowijo !)

licenseInfo.json

@@ -14,7 +14,6 @@
     "(MIT OR GPL-2.0)": ["editor/jquery-ui/jquery-ui-1.8.17.custom.min.js", "editor/jquerybbq/jquery.bbq.min.js", "editor/js-hotkeys/jquery.hotkeys.min.js"],
     "(MIT OR GPL-2.0-or-later)": ["editor/contextmenu/jQuery.contextMenu.js", "editor/extensions/ext-server_moinsave.js"],
     "Apache-2.0": ["editor/contextmenu.js", "editor/extensions/ext-foreignobject.js", "editor/extensions/ext-grid.js", "editor/extensions/ext-markers.js", "screencasts/svgopen2010/index.html", "editor/jgraduate/jQuery.jGraduate.js", "editor/extensions/mathjax/MathJax.min.js", "editor/extensions/mathjax/TeX-AMS-MML_SVG.js"],
-    "CC-BY-SA-2.0": ["editor/images/polygon.svg"],
     "ISC": ["editor/external/query-result/esm/index.js"],
     "LGPL-3.0-or-later": ["editor/jspdf/jspdf.plugin.svgToPdf.js"],
     "X11": ["editor/jspdf/jspdf.min.js"]