top: Remove a duplicate extern declaration for show_args.

This fixes a -Wnested-extern error with GCC 9. There is an existing
extern declaration in top.h.

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D31937

(cherry picked from commit 4c9cb057bd96bd10278f5ce7a735fff4c08e3c30)