xen/x86: Drop unnecessary barriers

xen/x86: Drop unnecessary barriers

x86's current implementation of wmb() is a compiler barrier.  As a result, the
only change in this patch is to remove an mfence instruction from
cpuidle_disable_deep_cstate().

None of these barriers serve any purpose.  They are not synchronising with
remote cpus, and their compiler-barrier properties are not needed for
correctness purposes.

Furthermore, these wmb()'s specifically do not want to turn into sfence
instructions in future changes where wmb()'s implementation is corrected.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>