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/bitcoin-tx.cpp

@@ -681,8 +681,6 @@ static void MutateTxSign(CMutableTransaction& tx, const std::string& flagStr)
 
 class Secp256k1Init
 {
-    ECCVerifyHandle globalVerifyHandle;
-
 public:
     Secp256k1Init() {
         ECC_Start();