Remove unused C++ code not covered by unit tests

src/key.cpp

@@ -131,14 +131,6 @@ void CKey::MakeNewKey(bool fCompressedIn) {
     fCompressed = fCompressedIn;
 }
 
-bool CKey::SetPrivKey(const CPrivKey &privkey, bool fCompressedIn) {
-    if (!ec_privkey_import_der(secp256k1_context_sign, (unsigned char*)begin(), &privkey[0], privkey.size()))
-        return false;
-    fCompressed = fCompressedIn;
-    fValid = true;
-    return true;
-}
-
 CPrivKey CKey::GetPrivKey() const {
     assert(fValid);
     CPrivKey privkey;