riscv: remove unused barrier defines

They were introduced in commit fab957c1 ("RISC-V: Atomic and
Locking Code") long after commit 2e39465a

 ("locking: Remove
deprecated smp_mb__() barriers") removed the remnants of all previous
instances from the tree.

Signed-off-by: Rolf Eike Beer <eb@emlix.com>
[paul.walmsley@sifive.com: stripped spurious mbox header from patch
 description; fixed commit references in patch header]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>