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-wallet.cpp

@@ -130,7 +130,6 @@ MAIN_FUNCTION
         return EXIT_FAILURE;
     }
 
-    ECCVerifyHandle globalVerifyHandle;
     ECC_Start();
     if (!wallet::WalletTool::ExecuteWalletToolFunc(args, command->command)) {
         return EXIT_FAILURE;