init_main: Remove the placeholder SYSINIT

This was initially introduced to ensure there is at least one entry so
that the sysinit_set symbol is not undefined. Well now that the kernel
has plenty of SYSINITs and it is unlikely we will have a kernel without
any SYSINITs in future, then this placeholder is not relevant anymore.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47916