sched/headers: Remove the 'init_pid_ns' prototype from <linux/sched.h>

pid.h already defines it.
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/sched.h

@@ -1301,8 +1301,6 @@ static inline struct thread_info *task_thread_info(struct task_struct *task)
 # define task_thread_info(task)	((struct thread_info *)(task)->stack)
 #endif
 
-extern struct pid_namespace init_pid_ns;
-
 /*
  * find a task by one of its numerical ids
  *