wallet: remove unused DescriptorScriptPubKeyMan::AddDescriptorKey

The private method has no callers. It opened a `WalletBatch` and
forwarded to `AddDescriptorKeyWithDB`, which is still called from two
other places.

Its last caller, in `CWallet::AddWalletDescriptor`, was replaced in
aa4f7823aa ("wallet: include keys when constructing DescriptorSPKM
during import"), which builds the manager with `CreateFromMigration`
instead of adding the key afterwards.

src/wallet/scriptpubkeyman.cpp

@@ -1173,15 +1173,6 @@ std::vector<WalletDestination> DescriptorScriptPubKeyMan::MarkUnusedAddresses(co
     return result;
 }
 
-void DescriptorScriptPubKeyMan::AddDescriptorKey(const CKey& key, const CPubKey &pubkey)
-{
-    LOCK(cs_desc_man);
-    WalletBatch batch(m_storage.GetDatabase());
-    if (!AddDescriptorKeyWithDB(batch, key, pubkey)) {
-        throw std::runtime_error(std::string(__func__) + ": writing descriptor private key failed");
-    }
-}
-
 bool DescriptorScriptPubKeyMan::AddDescriptorKeyWithDB(WalletBatch& batch, const CKey& key, const CPubKey &pubkey)
 {
     AssertLockHeld(cs_desc_man);

src/wallet/scriptpubkeyman.h

@@ -321,7 +321,6 @@ private:
 
     void Load();
 
-    void AddDescriptorKey(const CKey& key, const CPubKey &pubkey);
     void UpdateWithSigningProvider(WalletBatch& batch, const FlatSigningProvider& signing_provider) EXCLUSIVE_LOCKS_REQUIRED(cs_desc_man);
 
     //! Setup descriptors based on the given CExtKey