Remove bitfield check in `TreeNode` `preload`

[lejeunerenard]

The check will be performed in the core's download() regardless. The array of blocks are added as a range to the replicator which checks and compresses the range here: https://github.com/holepunchto/hypercore/blob/125aa810b54ff916f5b55cc21e3c17efb0b7ca5a/lib/replicator.js#L2668-L2669