arm/mpic: remove empty pic_init_secondary() hook

The default hook does nothing, so having an empty handler is pointless.
Simple cleanup.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D40475

(cherry picked from commit a8b2189c90c52e43e27614b2cca6b753a70da0ca)