repoobj: remove xxh64 checksums from blob header

[mr-raj12]

Description

The RepoObj blob header previously had two xxh64 checksums (meta and data). check_object() used them to detect byte-level corruption server-side without needing the borg key.

With SHA-256-named pack files, the server verifies a whole pack via its filename hash. Client-side AEAD decryption catches byte corruption too. The xxh64 checks are redundant given either of those.

Header shrinks from 24 bytes (<II8s8s) to 8 bytes (<II) per object.

Changes:

• repoobj.py: drop xxhash import, remove meta_hash/data_hash from struct and namedtuple, simplify format().
• repository.py: drop xxhash import, remove the two hash-check branches from check_object(). Size/truncation checks remain.
• check_cmd_test.py: test_verify_data no longer needs the fake_xxh64 workaround. test_corrupted_file_chunk now expects --repository-only to pass and --verify-data to catch corruption.
• repository_test.py, legacyrepository_test.py: drop xxhash import, update fchunk() to pack only sizes.

refs #8572

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.87%. Comparing base (b7c8417) to head (7f43779).
⚠️ Report is 13 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9677      +/-   ##
==========================================
- Coverage   83.99%   83.87%   -0.12%     
==========================================
  Files          93       93              
  Lines       15592    15586       -6     
  Branches     2337     2335       -2     
==========================================
- Hits        13096    13073      -23     
- Misses       1771     1785      +14     
- Partials      725      728       +3     

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[ThomasWaldmann]

LGTM.