sched/fair: Clean up comment in nohz_idle_balance()

sched/fair: Clean up comment in nohz_idle_balance()

Concerning the comment associated to the atomic_fetch_andnot() in
nohz_idle_balance(), Vincent explains [1]:

  "[...] the comment is useless and can be removed [...]  it was
   referring to a line code above the comment that was present in
   a previous iteration of the patchset. This line disappeared in
   final version but the comment has stayed."

So remove the comment.

Vincent also points out that the full ordering associated to the
atomic_fetch_andnot() primitive could be relaxed, but this patch
insists on the current more conservative/fully ordered solution:

"Performance" isn't a concern, stay away from "correctness"/subtle
relaxed (re)ordering if possible..., just make sure not to confuse
the next reader with misleading/out-of-date comments.

[1] http://lkml.kernel.org/r/CAKfTPtBjA-oCBRkO6__npQwL3+HLjzk7riCcPU1R7YdO-EpuZg@mail.gmail.com

Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20181127110110.5533-1-andrea.parri@amarulasolutions.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>