RISC-V: Remove Merkleisable trait

Closes RV-414.

What

Follow-up from !16378 (merged).

This MR removes Merkleisable completely, after its functionality was moved to the ProofLayout trait. The ProofLayout::to_proof function is also renamed back to to_merkle_tree.

Next: !16294 (merged) removes the RootHashable trait

Why

This trait is no longer needed.

How

Removing Merkleisable has required some changes to the tests of the proof-generating backend.

Manually Testing

make -C src/riscv all

Benchmarking

No impact on performance.

Tasks for the Author

• Link all Linear issues related to this MR using magic words (e.g. part of, relates to, closes).
• Eliminate dead code and other spurious artefacts introduced in your changes.
• Document new public functions, methods and types.
• Make sure the documentation for updated functions, methods, and types is correct.
• Add tests for bugs that have been fixed.
• Put in reasonable effort to ensure that CI will pass.
  • make -C src/riscv
  • dune build src/lib_riscv
  • dune build src/rust_deps
• Benchmark performance and populate the table above if needed.
• Explain changes to regression test captures when applicable.
• Write commit messages to reflect the changes they're about.
• Self-review your changes to ensure they are high-quality.
• Complete all of the above before assigning this MR to reviewers.