Delete synctex file upon latexmk -C

latexmk -c deletes all unnecessary compiler output (log, aux, toc, etc.
pp.), -C additionally deletes the “actual” output, i.e. DVI and PDF. The
synctex file is useless without the PDF, so delete it as well.

Signed-off-by: Thomas Schneider <thosch97@gmail.com>