Remove or rename xxHash.txt

[cbenhagen]

Either add date / time to the filename or completely remove the file as the information it contains is redundant.

[cbenhagen]

Fixed by 4b3508f — xxHash.txt is no longer written. Per-file xxh64 digests now live in the ASC MHL generation manifests (or, when --legacy-mhl is passed, in the flat MHL v1.1 file).