chore: remove FFI layer

[xdustinface]

Delete the dash-spv-ffi and key-wallet-ffi crates and remove the optional ffi feature, ffi module, build.rs, and cbindgen.toml from dash-network. It is no longer used by anything. Keeping it around for potential future use just eats time and tokens to keep it in sync with the Rust API, make sure it's tested and working, and review all the changes happening to it.

I think it makes more sense to just generate a new FFI layer if we ever come up with a use case or if someone actually requests it. If someone is actually using it they will probably look into here and are welcome to open an issue, so we can consider reverting the PR.

But at the moment all the effort we put into this is better put into something else.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.31%. Comparing base (26384af) to head (c2460a7).
⚠️ Report is 1 commits behind head on v0.42-dev.

Additional details and impacted files

@@              Coverage Diff              @@
##           v0.42-dev     #776      +/-   ##
=============================================
+ Coverage      72.66%   76.31%   +3.65%     
=============================================
  Files            320      291      -29     
  Lines          70319    60846    -9473     
=============================================
- Hits           51098    46437    -4661     
+ Misses         19221    14409    -4812     

Flag	Coverage Δ
core	76.30% <ø> (ø)
ffi	?
rpc	20.00% <ø> (ø)
spv	89.80% <ø> (-0.04%)	⬇️
wallet	71.27% <ø> (ø)
see 2 files with indirect coverage changes

[ZocoLini]

Sam currently doesn't want to remove the KeyWallet wrappers, he is worried about a hypothetical future swift consumer that wants the key derivation features so key-wallet-ffi and dash-network/ffi cannot be removed. I honestly think that focusing in what we need first is a better idea than trying to maintain ffi layers that we don't internally use

[github-actions]

This PR has merge conflicts with the base branch. Please rebase or merge the base branch into your branch to resolve them.