btrfs: remove btrfs_bit_radix_cachep declaration

btrfs_bit_radix_cachep has been removed since
commit 45c06543 ("Btrfs: remove unused btrfs_bit_radix slab"),
so remove it.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -42,7 +42,6 @@ struct btrfs_delayed_ref_root;
 struct btrfs_space_info;
 struct btrfs_block_group;
 extern struct kmem_cache *btrfs_trans_handle_cachep;
-extern struct kmem_cache *btrfs_bit_radix_cachep;
 extern struct kmem_cache *btrfs_path_cachep;
 extern struct kmem_cache *btrfs_free_space_cachep;
 extern struct kmem_cache *btrfs_free_space_bitmap_cachep;