sched: Remove unused variable in 'struct sched_domain'

The 'u64 last_update' variable isn't used now, remove it to save a bit of space.

Signed-off-by: Alex Shi <alex.shi@linaro.org>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Morten.Rasmussen@arm.com
Cc: linaro-kernel@lists.linaro.org
Link: http://lkml.kernel.org/r/1384852912-24791-1-git-send-email-alex.shi@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>