wallet, rpc: remove obsolete "keypoololdest" result field/code

This `getwalletinfo()` result field was only ever returned for
legacy wallets and is hence not relevant anymore, so we can
delete it and the corresponding CWallet/ScriptPubKeyMan code
behind it.

src/wallet/scriptpubkeyman.cpp

@@ -1189,13 +1189,6 @@ bool DescriptorScriptPubKeyMan::HaveCryptedKeys() const
     return !m_map_crypted_keys.empty();
 }
 
-std::optional<int64_t> DescriptorScriptPubKeyMan::GetOldestKeyPoolTime() const
-{
-    // This is only used for getwalletinfo output and isn't relevant to descriptor wallets.
-    return std::nullopt;
-}
-
-
 unsigned int DescriptorScriptPubKeyMan::GetKeyPoolSize() const
 {
     LOCK(cs_desc_man);