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>