xdiff-interface: drop parse_hunk_header()

xdiff-interface: drop parse_hunk_header()

This function was used only for parsing the hunk headers generated by
xdiff. Now that we can use hunk callbacks to get that information
directly, it has outlived its usefulness.

Note to anyone who wants to resurrect it: the "len" parameter was
totally unused, meaning that the function could read past the end of the
"line" array. In practice this never happened, because we only used it
to parse xdiff's generated header lines. But it would be dangerous to
use it for other cases without fixing this defect.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>