#37362 Delete the unused draft FFI host function implementation

What this does

ffi/host_fns.rs was an earlier draft of generate_symbols / generate_symbol_for_function / Function::print_source_code that ffi/mod.rs still declared and re-exported, but the live implementations are the ones in ffi_body.rs, which is what every caller uses. This deletes the draft and the Function/Step/Compiled struct copies in mod.rs that only it referenced.

Split 5 of 8 from #32023, which alii asked to be broken up. Pure code motion and deletion; no intended behavior change. Stacked on claude/split/install-cli (merge order: foundations → install-cli → these, in any order among themselves).

Verification

• Compiles in isolation against the base (cargo check), and the union of all eight splits is byte-identical to #32023's head f29ce8fe4, which CI ran at 285/286 green (the one red was the fleet-wide napi.test.ts GC-timing flake on windows x64-baseline).
• Locally on the parent head: ffi.test.js passes. The deleted file had no callers (grep for its exports resolves only to the ffi_body definitions).
• No new tests: this adds no behavior, so the existing suites above are the regression coverage.