sh: fix smp_store_mb for !SMP

sh variant of smp_store_mb() calls xchg() on !SMP which is stronger than
implied by both the name and the documentation.

commit 90a3ccb0 ("sh: define __smp_xxx,
fix smp_store_mb for !SMP") was supposed to fix it but
left the bug in place.

Drop smp_store_mb, so that code in asm-generic/barrier.h
will define it correctly depending on CONFIG_SMP.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>