Remove include of grffile from default latex template.

This package is needed for proper handling of image filenames
containing periods (in addition to the period before the
extension).

Unfortunately, grffile breaks in the latest texlive update.
Until a fix is released (see ho-tex/oberdiek#73) it seems best
to remove this from the default template.

This may cause problems if you have filenames with periods.
The workaround is to put `\usepackage{grffile}` in header-includes,
and be sure you're using an older version of texlive packages.

See #5848. We will leave that issue open to remind us to
check upstream, and restore grffile when it's possible to
do so.

data/templates/default.latex

@@ -270,7 +270,7 @@ $else$
 $endif$
 $endif$
 $if(graphics)$
-\usepackage{graphicx,grffile}
+\usepackage{graphicx}
 \makeatletter
 \def\maxwidth{\ifdim\Gin@nat@width>\linewidth\linewidth\else\Gin@nat@width\fi}
 \def\maxheight{\ifdim\Gin@nat@height>\textheight\textheight\else\Gin@nat@height\fi}

test/writer.latex

@@ -43,7 +43,7 @@
   pdfcreator={LaTeX via pandoc}}
 \urlstyle{same} % disable monospaced font for URLs
 \VerbatimFootnotes % allow verbatim text in footnotes
-\usepackage{graphicx,grffile}
+\usepackage{graphicx}
 \makeatletter
 \def\maxwidth{\ifdim\Gin@nat@width>\linewidth\linewidth\else\Gin@nat@width\fi}
 \def\maxheight{\ifdim\Gin@nat@height>\textheight\textheight\else\Gin@nat@height\fi}