Remove TODOs for ffi functions for setting secp256k1 callbacks
Remove TODOs for ffi functions for setting secp256k1 callbacks

We would not want to use these functions internally because we rely on
USE_EXTERNAL_DEFAULT_CALLBACKS to provide the callbacks at link time,
see f7a4a7ef577d52422571bef8dbc862c573077f09. Moreover, we would not
want to export the functions either.

apoelstra/rust-secp256k1,apoelstra/rust-secp256k1,apoelstra/rust-secp256k1,apoelstra/rust-secp256k1