Remove unused sfield.rs

[sappenin]

High Level Overview of Change

Remove unused sfield.rs

Type of Change

• Refactor (non-breaking change that only restructures code)

[mvadari]

Where do we get SFields from if not there?

[sappenin]

Where do we get SFields from if not there?

They're defined in the xrpl-std-lib for all WASM usage here. The wasm-host-simulator has a dependency on that (i.e., on xrpl-std-lib), and if anything in the simulator needs sfields, it can source them from there.

Of course, this does create a dependency which maybe we shouldn't have (this PR doesn't introduce this dependency, FWIW). I'd be open to that argument if you or @pwang200 think we should have duplicate files that have the same sfield declarations?