arm64, riscv: remove unused declaration

It is inherited from arm, where the global exists and is used. No
functional change.

Reviewed by: markj
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45323

(cherry picked from commit 1d3c23676de33762fd7fc2e3d890fd14738d3ee6)