sys/sys/kernel.h: Remove SI_SUB_DONE
It was not used since 9a7add6d01f3.
Reviewed by: mhorne, imp
Fixes: 9a7add6d01f3 init_main: Switch from sysinit array to SLIST
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47915
(cherry picked from commit 81f9d3b11f310b28e4bf8cc2ad2d84e4aadd929d)