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/bench/verify_script.cpp

@@ -18,7 +18,6 @@
 // modified to measure performance of other types of scripts.
 static void VerifyScriptBench(benchmark::Bench& bench)
 {
-    const ECCVerifyHandle verify_handle;
     ECC_Start();
 
     const uint32_t flags{SCRIPT_VERIFY_WITNESS | SCRIPT_VERIFY_P2SH};