Remove unused C++ code not covered by unit tests

src/key.h

@@ -94,9 +94,6 @@ public:
     //! Check whether the public key corresponding to this private key is (to be) compressed.
     bool IsCompressed() const { return fCompressed; }
 
-    //! Initialize from a CPrivKey (serialized OpenSSL private key data).
-    bool SetPrivKey(const CPrivKey& vchPrivKey, bool fCompressed);
-
     //! Generate a new private key using a cryptographic PRNG.
     void MakeNewKey(bool fCompressed);