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/kernel/context.cpp

@@ -21,12 +21,10 @@ Context::Context()
     LogPrintf("Using the '%s' SHA256 implementation\n", sha256_algo);
     RandomInit();
     ECC_Start();
-    ecc_verify_handle.reset(new ECCVerifyHandle());
 }
 
 Context::~Context()
 {
-    ecc_verify_handle.reset();
     ECC_Stop();
 }