Adapt to libsecp256k1 API changes

* Use SECP256K1_CONTEXT_NONE when creating signing context, as
  SECP256K1_CONTEXT_SIGN is deprecated and unnecessary.
* Use secp256k1_static_context where applicable.

src/test/fuzz/message.cpp

@@ -18,7 +18,6 @@
 
 void initialize_message()
 {
-    static const ECCVerifyHandle ecc_verify_handle;
     ECC_Start();
     SelectParams(CBaseChainParams::REGTEST);
 }