stand: Move MOD_xxx macros from modinfo.h to .c

Now that MOD_xxx macros are modinfo.c, they don't need to be in
modinfo.h.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36573

(cherry picked from commit 2e6ed47a4609ff03a9308a173c64900485172c22)