wallet/migration: use HavePrivateKeys in place of ToPrivateString

ToPrivateString() behaviour will be modified in the following commits.

In order to keep the scope of this PR limited to the RPC behaviour,
this commit updates wallet migration to use 'Descriptor::HavePrivateKeys()'
in place of 'Descriptor::ToPrivateString()' to determine watchonly descriptors.

A follow-up PR can be opened to update migration logic to exclude
descriptors with some private keys from the watchonly migration wallet.

src/wallet/scriptpubkeyman.cpp

@@ -717,10 +717,9 @@ std::optional<MigrationData> LegacyDataSPKM::MigrateToDescriptor()
 
         std::vector<CScript> desc_spks;
 
-        // Make the descriptor string with private keys
+        // If we can't provide all private keys for this inferred descriptor,
-        std::string desc_str;
+        // but this wallet is not watch-only, migrate it to the watch-only wallet.
-        bool watchonly = !desc->ToPrivateString(*this, desc_str);
+        if (!desc->HavePrivateKeys(*this) && !m_storage.IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
-        if (watchonly && !m_storage.IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
             out.watch_descs.emplace_back(desc->ToString(), creation_time);
 
             // Get the scriptPubKeys without writing this to the wallet