webui: remove outdated Readme from v7 canvaspainter
webui: remove outdated Readme from v7 canvaspainter

Its content was moved to cefdisplay and qt5webdisplay
olifre/root,karies/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,karies/root,karies/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,olifre/root,karies/root,karies/root,olifre/root,olifre/root,karies/root,olifre/root,karies/root,root-mirror/root