stand: Retire setting hw.eisa_slots.

When the eisa code was removed in 2017, prior to the stable/12 branch,
setting hw.eisa_slots became a nop. The oldest supported branch doesn't
have eisa at all. The need to set it manually on boot disappeared
largely by 2000...

Sponsored by: Netflix

(cherry picked from commit 21795c374aceb685dbdb1bd18c7a5c41c3cf0baf)