LaTeX export: grffile is a stub package

[meedstrom]

According to https://ctan.org/pkg/grffile, since 2019-11-08, grffile is a stub 
that just loads graphicx, part of texlive-latex-graphics, part of texlive-base. 
 My system (Guix) doesn't have a package for grffile, so I can't generate a 
PDF, which is silly because it's a stub anyway.

Suggest fixing by removing \usepackage{grffile} in LaTeX export.

A caveat: some distributions seem to still package an old TeXLive full 
distribution, going by Guix which provides a three-gigabyte "texlive" package 
from 2019-04-10.  I'm not sure what sort of deprecation time we allow.

Martin Edström