locking/lockdep: Cleanup the repeated declaration

'struct task_struct' has been decleared twice, so keep the top one and
cleanup the repeated one.
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/1629875224-32751-1-git-send-email-zhangshaokun@hisilicon.com

include/linux/debug_locks.h

@@ -47,8 +47,6 @@ extern int debug_locks_off(void);
 # define locking_selftest()	do { } while (0)
 #endif
 
-struct task_struct;
-
 #ifdef CONFIG_LOCKDEP
 extern void debug_show_all_locks(void);
 extern void debug_show_held_locks(struct task_struct *task);