This PR reverts these commit and PR:

• Revert deb2f06 mina_block/tests: use json file instead of OCaml module
• Revert Parse old precomputed blocks correctly by allowing Field.t to be deserialized from decimals #17099

The parsing feature won't be needed anymore in Mesa HF, hence no need to keep the code.

And also part of PR #17115.

Motivation for the change:

1. We're going to change the block format, so the test relying on an old block from devnet will fail
2. We'd need to artificially recreate the test file
3. The fallback that is reverted here won't be needed anymore, because from Mesa HF onwards there won't be any block using decimal format for the authorization_kind field

Explain how you tested your changes:

• Relying on an existing test suite

Checklist:

• Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• Modified the current draft of release notes with details on what is completed or incomplete within this project
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? None