alpha: kill unnecessary __used attribute in PER_CPU_ATTRIBUTES

With the previous percpu variable definition change, all percpu
variables are global and there's no need to specify __used, which only
triggers on recent compilers anyway.  Kill it.

[ Impact: remove unnecessary percpu attribute ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Richard Henderson <rth@twiddle.net>