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/psbt.cpp

@@ -22,12 +22,7 @@ using node::AnalyzePSBT;
 using node::PSBTAnalysis;
 using node::PSBTInputAnalysis;
 
-void initialize_psbt()
-{
-    static const ECCVerifyHandle verify_handle;
-}
-
-FUZZ_TARGET_INIT(psbt, initialize_psbt)
+FUZZ_TARGET(psbt)
 {
     FuzzedDataProvider fuzzed_data_provider{buffer.data(), buffer.size()};
     PartiallySignedTransaction psbt_mut;