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/script/bitcoinconsensus.cpp

@@ -62,12 +62,6 @@ inline int set_error(bitcoinconsensus_error* ret, bitcoinconsensus_error serror)
     return 0;
 }
 
-struct ECCryptoClosure
-{
-    ECCVerifyHandle handle;
-};
-
-ECCryptoClosure instance_of_eccryptoclosure;
 } // namespace
 
 /** Check that all specified flags are part of the libconsensus interface. */