smp: remove unused hlt_cpus_mask

It is a relic, apparently once populated by a machdep.hlt_cpus sysctl.
The sysctl was removed, and ULE has never honored this mask. It is now
safe to remove.

Remove the mask, and its few remaining references in: sched_4bsd(4),
hwpmc(4), and hwt(4).

Reviewed by: olce, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58158