files: riscv, arm64: Remove redundant 'ofw_cpu.c'

Should have been removed when that line was moved from 'files.arm' to
'files'.

Fixes: 14e1a2cd295d ("Move ofw_cpu file to the main files conf file.")
MFC after: 2 weeks
Event: Halifax Hackathon 202606
Location: Seat 36K in AC667, over Maine near Canadian border
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 10213f01773f22ab948ec1e87c880b1d19a1fc45)