sched: Remove struct rq::nohz_stamp

sched: Remove struct rq::nohz_stamp

The nohz_stamp member of struct rq has been unused since 2010,
when this commit removed the code that referenced it:

  396e894d289d ("sched: Revert nohz_ratelimit() for now")

Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20160815121410.5ea1c98f@annuminas.surriel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>