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.

Sponsored by: Netflix

(cherry picked from commit 67d2bd974d34d918255307321107273562f11dde)