shared: remove unused function strrpart()

cgit.h

@@ -334,7 +334,6 @@ extern int chk_non_negative(int result, char *msg);
 extern char *trim_end(const char *str, char c);
 extern char *ensure_end(const char *str, char c);
 extern char *strlpart(char *txt, int maxlen);
-extern char *strrpart(char *txt, int maxlen);
 
 extern void strbuf_ensure_end(struct strbuf *sb, char c);