stream-restore: Drop the version field from the entry struct

stream-restore: Drop the version field from the entry struct

Storing the version in the entry struct is pointless. We should always
write entries using the current version. When we encounter older
versions when reading, those need to be converted to the current version
anyway, because all code that uses the entry struct assumes that the
data is stored according to the current version semantics.

We're currently at the first version of the database entries, so
currently there's no version conversion happening. I have a patch that
will increment the entry version, so this is preparation for that.