btrfs: tree-checker: remove duplicate definition of 'inode_item_err'

Remove the duplicate definition of 'inode_item_err' in the file
tree-checker.c that got there by accident in c23c77b0 ("btrfs:
tree-checker: Refactor inode key check into seperate function").

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Zheng Wei <wei.zheng@vivo.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>