percpu: remove this_cpu_xor() implementation

There is not a single user in the whole kernel.
Besides that this_cpu_xor() is broken anyway since it gets
translated to this_cpu_or() (see __pcpu_size_call() line).

So instead of fixing an unused definition just remove it.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>