I checked my 2 Bitcoin Core installations (on different systems): The size of blocks is 510671104 and 509643300 kB (the difference is 1027804 kB). The oldest installation is the largest: it makes sense the more recent installation skipped all old orphaned blocks.
if it's useless, can we delete it manually on the block file?
It's not just one file, it's part of other files. So you can't delete it. And it wouldn't make much difference: you'll save 0.2% disk space, which will be filled again 3 days later.
is it dangerous if I edit manually (delete) blocks (that almost 7K file)?
Bitcoin Core assumes the files on your disk are correct. If you delete some of it, you can expect unpredictable behaviour (such as crashes). It's not worth it.