bc: Delete md5 hashing from restore

[georgimld]

Summary 💡

When creating a backup, a hashing is now calculated with a sha256 hash. It used to be created with md5, but as mentioned in #513 it wasn't working perfectly. Restoring a backup is now possible with both the old version (md5 hash) and the new (sha256), as implemented in #693 .
Deleting the md5 hash function would not allow old backups to be restored anymore, this would be a breaking change and should only be implemented in Trubudget 2.0.0.