stand: Retire setting hw.ata.wc: it doesn't exist.

hw.ata.wc was disconnected as part ot the 2013 cam-ification of ata. No
need to continue setting it. It's been unused in FreeBSD 10.x and newer.

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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...

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Remove $FreeBSD$: one-line forth tag

Remove /^\\[\s*]*\$FreeBSD\$.*$\n/

Move sys/boot to stand. Fix all references to new location

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