ui-shared: remove toggle_ssdiff arg to cgit_diff_link()

This argument is never used with a value other than zero, so remove it
and simplify the code.

Signed-off-by: John Keeping <john@keeping.me.uk>
diff --git a/ui-shared.h b/ui-shared.h
index bb193ec..fc9be3b 100644
--- a/ui-shared.h
+++ b/ui-shared.h
@@ -43,7 +43,7 @@
 extern void cgit_diff_link(const char *name, const char *title,
 			   const char *class, const char *head,
 			   const char *new_rev, const char *old_rev,
-			   const char *path, int toggle_ssdiff);
+			   const char *path);
 extern void cgit_stats_link(const char *name, const char *title,
 			    const char *class, const char *head,
 			    const char *path);