RISC-V: Abolish allocate on Layouts

Part of RV-568

Stacked MRs

• RISC-V: Initialise Buddy branch correctly (!17570 - merged)
• RISC-V: Abolish `allocate` on `Layouts` (!17566 - merged) (you are here)
• RISC-V: Prefer `new` over `allocate` with `bind` (!17569 - merged)
• RISC-V: New state allocation and initialisation... (!17567 - merged)

What

Remove the allocate method from the Layout trait.

Why

It is no longer used. Allocation and initialisation happens through the state data types directly.

Please refer to !17567 (merged) for more context!

Manually Testing

make -C src/riscv all

Benchmarking

Runtime performance is unchanged.

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.