Remove `SECP256K1_WARN_UNUSED_RESULT` from `secp256k1_keypair_*` functions #1379

Why do secp256k1_keypair_* functions have SECP256K1_WARN_UNUSED_RESULT if they always return 1 ? The only exception is secp256k1_keypair_create.

Why do secp256k1_keypair_* functions have SECP256K1_WARN_UNUSED_RESULT if they always return 1 ?

Because they are not guaranteed to always return 1 in future (but backwards-compatible) API versions, I suppose.

I don't think that's a concern. If a backward-compatible API change causes 0 to become a permitted return value, it shouldn't occur for code written against the old header file (e.g. it'd require API calls, or constants, or magics, ... that don't exist in the old header). Thus, to any user of the old header file, no warning should be needed, and the WARN_UNUSED_RESULT can just be added to the header whenever such an API change is introduced.

TL;DR: I think we should drop SECP256K1_WARN_UNUSED_RESULT for functions that are defined to only return 1.

These functions can actually return 0, but this requires a violation of the API: https://github.com/bitcoin-core/secp256k1/blob/b40e2d30b7a6f71e7d82340c1cf82150c5cf0540/src/modules/extrakeys/main_impl.h#L44-L54

Wouldn't it make more sense to call the illegal callback in that case?

@real-or-random secp256k1_xonly_pubkey_load calls the illegal callback if it fails.

Changelog was added in #1702. Dropping "needs-changelog".