[dev-context] removing .pgf file with --purgeall

Hello Hans, may I request removing \jobname.pgf when doing context/ctxtools --purgeall (in both mkii/mkiv)? That's a file that TikZ creates by default (similar to \jobname-mpgraph*, only serving a different purpose) and is only needed when a document is compiled on computer without tikz installed. (Low priority request.) Mojca