Remove ocaml-interop dependency and associated FFI code

[Copilot]

This PR removes the fork of ocaml-interop from sebastiencs and the associated FFI code as requested in the issue. Following @tizoc's comment that the dependency can be removed entirely as it's a remnant from very long ago, this change cleanly removes all OCaml interoperability code.

Changes Made

1. Removed ocaml-interop dependency from ledger/Cargo.toml:

   • Removed the git dependency pointing to sebastiencs fork
   • Removed commented alternative dependencies

2. Removed conditional FFI module from ledger/src/lib.rs:

   • Removed the feature-gated mod ffi; declaration

3. Deleted entire FFI directory ledger/src/ffi/:

   • block.rs - OCaml FFI bindings for block operations
   • database.rs - OCaml FFI bindings for database operations
   • mask.rs - OCaml FFI bindings for mask operations
   • ondisk.rs - OCaml FFI bindings for ondisk operations
   • mod.rs - FFI module declarations

Verification

• ✅ Build succeeds without the dependency
• ✅ Tests continue to pass
• ✅ The ocaml-interop feature is no longer available
• ✅ No remaining references to ocaml-interop in the codebase
• ✅ Code formatting and style checks pass

The removal is clean and safe since the FFI code was only conditionally compiled when the ocaml-interop feature was explicitly enabled, and the main codebase never relied on it.

Fixes #3103671163.

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[dannywillems]

Do not merge. This is a test.