Remove transaction_witness.mli

[georgeee]

This interface file is an exact copy of the transaction_witness.ml, hence it's redundant to keep it.

Explain how you tested your changes:

• An extra-simple refactoring, and it compiles

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

[georgeee]

!ci-build-me

[dannywillems]

Don't we want to keep the mli and modify it step by step? It's been added automatically i guess using ocamlc. Always good to have interfaces!

[mrmr1993]

It's generally better to remove the .ml file instead, and mark it as such in dune. Implementation compilation is more expensive than interface compilation, so this is likely to slow down the build process.