Retire diffcore-pathspec.

Nobody except diff-stages used it -- the callers instead filtered
the input to diffcore themselves.  Make diff-stages do that as
well and retire diffcore-pathspec.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.h

@@ -20,7 +20,6 @@ typedef void (*add_remove_fn_t)(struct diff_options *options,
 		    const char *base, const char *path);
 
 struct diff_options {
-	const char **paths;
 	const char *filter;
 	const char *orderfile;
 	const char *pickaxe;