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

[Mojca Miklavec]

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