sched/core: Remove unnecessary unlikely() in push_*_task()

WARN_ON() already contains an unlikely(), so it's not necessary to
use WARN_ON(1).

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20181103172602.1917-1-tiny.windzz@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>