8801e319d5 refactor: remove unused `CKey::Negate` method (Sebastian Falbesoner)
Pull request description:
This method was introduced as a pre-requirement for the v2 transport protocol back then (see PR #14047, commit 463921bb), when it was still BIP151. With the replacement BIP324, this is not needed anymore, and it's also unlikely that for any other proposal we'd ever need to negate private keys at this abstraction level. I'd argue that this operation is usually something that should happen within a secp256k1 module (like e.g. done in MuSig2, Silent Payments...).
(If there is really demand in the future, it's also trivial to reintroduce the method.)
ACKs for top commit:
laanwj:
ACK 8801e319d5
sipa:
ACK 8801e319d5
achow101:
ACK 8801e319d5
Tree-SHA512: 7bc1566399635c5c6e4940a2724c865d5443eb190024379099330c023c516f1e4f423ed9e8c42bc93413b723a5464ec79d3f879f58c0e598fe24f495238df4ec
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