buf_ring: Remove PREFETCH_DEFINED

I'm not able to find anything in the tree that ever defined it. Remove
as it's unused so is untested.

Reviewed by: alc, imp, kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46148

(cherry picked from commit 5048308bdb76f40e88c9133658fc61d82158ded2)