diff: remove "diff-files -q" in a version of Git in a distant future

This was inherited from "show-diff -q" that was invented to tell
comparison between the index and the working tree to ignore only
removals in 2005.

These days, it is spelled as "--diff-filter=d".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.h

@@ -341,8 +341,6 @@ extern int parse_rename_score(const char **cp_p);
 
 extern long parse_algorithm_value(const char *value);
 
-extern void handle_deprecated_show_diff_q(struct diff_options *);
-
 extern int print_stat_summary(FILE *fp, int files,
 			      int insertions, int deletions);
 extern void setup_diff_pager(struct diff_options *);