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/checkqueue.cpp

@@ -25,7 +25,6 @@ static void CCheckQueueSpeedPrevectorJob(benchmark::Bench& bench)
     // We shouldn't ever be running with the checkqueue on a single core machine.
     if (GetNumCores() <= 1) return;
 
-    const ECCVerifyHandle verify_handle;
     ECC_Start();
 
     struct PrevectorJob {