SharedTorrent: The storage bucket isn't authoritative for size

[shevek]

https://github.com/shevek/ttorrent/commit/2d3008574a00a27e2acef0b67015b6d9b5245bcb

I'm writing to a storage layer which doesn't support size, but that led me to discover this. It doesn't matter how large the store is, only how large the torrent data ought to be, and the store just has to deal with whatever (torrent-valid) ranges the engine might send to it. With this patch, the size() method can even be removed from the TorrentByteStorage interface, as nothing calls it.