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_assets_test_minimizer.cpp

@@ -184,10 +184,7 @@ void Test(const std::string& str)
     }
 }
 
-void test_init()
-{
-    static ECCVerifyHandle handle;
-}
+void test_init() {}
 
 FUZZ_TARGET_INIT_HIDDEN(script_assets_test_minimizer, test_init, /*hidden=*/true)
 {