refactor: remove unused CKey::Negate method

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 any other proposal would need to negate private
keys at this abstraction level.
(If there is really demand, it's trivial to reintroduce the method.)

src/key.cpp

@@ -166,12 +166,6 @@ void CKey::MakeNewKey(bool fCompressedIn) {
     fCompressed = fCompressedIn;
 }
 
-bool CKey::Negate()
-{
-    assert(keydata);
-    return secp256k1_ec_seckey_negate(secp256k1_context_sign, keydata->data());
-}
-
 CPrivKey CKey::GetPrivKey() const {
     assert(keydata);
     CPrivKey seckey;