hgweb: drop support of browsers that don't understand <canvas> (BC) Internet Explorer 8 and below need excanvas aka ExplorerCanvas to support canvas tag at all. We used to vendor the library in Mercurial, but IE8 was discontinued and all support for it ceased in January 2016. We should do the same and remove excanvas library. Apart from just cleaning up code, this will also make downstream Debian packages stop depending on libjs-excanvas, for example.