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/script.cpp

@@ -32,9 +32,6 @@
 
 void initialize_script()
 {
-    // Fuzzers using pubkey must hold an ECCVerifyHandle.
-    static const ECCVerifyHandle verify_handle;
-
     SelectParams(CBaseChainParams::REGTEST);
 }