Remove an unused arm64 panic string

This was added early in the development of the arm64 port when
cpu_switch was just a stub. It should have been removed when cpu_switch
was implemented, however this didn't seem to be the case, and the '%p'
was added.

As this hasn't been needed in 7 years we can remove it.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 267d55fa2a74a6e04eb760e4cbb07c7f66729ac9)