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/descriptor_parse.cpp

@@ -9,7 +9,6 @@
 
 void initialize_descriptor_parse()
 {
-    static const ECCVerifyHandle verify_handle;
     ECC_Start();
     SelectParams(CBaseChainParams::MAIN);
 }