sched/core: Remove mmdrop() definition

Commit
   bf2c59fce4 ("sched/core: Fix illegal RCU from offline CPUs")

introduced a definition for mmdrop() but a a few lines above there is
already mmdrop() defined as static inline.

Remove the newly introduced mmdrop() definition.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200618190810.790211-1-bigeasy@linutronix.de

include/linux/sched/mm.h

@@ -49,8 +49,6 @@ static inline void mmdrop(struct mm_struct *mm)
 		__mmdrop(mm);
 }
 
-void mmdrop(struct mm_struct *mm);
-
 /*
  * This has to be called after a get_task_mm()/mmget_not_zero()
  * followed by taking the mmap_lock for writing before modifying the