buf_ring: Remove old arm-only dequeue code

In the single consumer dequeue the consumer thread controls
br_cons_head. As such no ordering between this and other data are
required.

Reviewed by: alc, imp, kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46153

(cherry picked from commit 7eb0fffc7792bc294d6b86546a3923e2e2f83f8a)