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/rpc/wallet.cpp

@@ -46,7 +46,6 @@ static RPCHelpMan getwalletinfo()
                         {RPCResult::Type::STR_AMOUNT, "unconfirmed_balance", "DEPRECATED. Identical to getbalances().mine.untrusted_pending"},
                         {RPCResult::Type::STR_AMOUNT, "immature_balance", "DEPRECATED. Identical to getbalances().mine.immature"},
                         {RPCResult::Type::NUM, "txcount", "the total number of transactions in the wallet"},
-                        {RPCResult::Type::NUM_TIME, "keypoololdest", /*optional=*/true, "the " + UNIX_EPOCH_TIME + " of the oldest pre-generated key in the key pool. Legacy wallets only."},
                         {RPCResult::Type::NUM, "keypoolsize", "how many new keys are pre-generated (only counts external keys)"},
                         {RPCResult::Type::NUM, "keypoolsize_hd_internal", /*optional=*/true, "how many new keys are pre-generated for internal use (used for change outputs, only appears if the wallet is using this feature, otherwise external keys are used)"},
                         {RPCResult::Type::NUM_TIME, "unlocked_until", /*optional=*/true, "the " + UNIX_EPOCH_TIME + " until which the wallet is unlocked for transfers, or 0 if the wallet is locked (only present for passphrase-encrypted wallets)"},
@@ -91,10 +90,6 @@ static RPCHelpMan getwalletinfo()
     obj.pushKV("unconfirmed_balance", ValueFromAmount(bal.m_mine_untrusted_pending));
     obj.pushKV("immature_balance", ValueFromAmount(bal.m_mine_immature));
     obj.pushKV("txcount",       (int)pwallet->mapWallet.size());
-    const auto kp_oldest = pwallet->GetOldestKeyPoolTime();
-    if (kp_oldest.has_value()) {
-        obj.pushKV("keypoololdest", kp_oldest.value());
-    }
     obj.pushKV("keypoolsize", (int64_t)kpExternalSize);
 
     if (pwallet->CanSupportFeature(FEATURE_HD_SPLIT)) {

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);

src/wallet/scriptpubkeyman.h

@@ -133,8 +133,6 @@ public:
     //! The action to do when the DB needs rewrite
     virtual void RewriteDB() {}
 
-    virtual std::optional<int64_t> GetOldestKeyPoolTime() const { return GetTime(); }
-
     virtual unsigned int GetKeyPoolSize() const { return 0; }
 
     virtual int64_t GetTimeFirstKey() const { return 0; }
@@ -364,7 +362,6 @@ public:
     std::optional<CKey> GetKey(const CKeyID& keyid) const EXCLUSIVE_LOCKS_REQUIRED(cs_desc_man);
     bool HaveCryptedKeys() const override;
 
-    std::optional<int64_t> GetOldestKeyPoolTime() const override;
     unsigned int GetKeyPoolSize() const override;
 
     int64_t GetTimeFirstKey() const override;

src/wallet/wallet.cpp

@@ -2506,20 +2506,6 @@ util::Result<CTxDestination> CWallet::GetNewChangeDestination(const OutputType t
     return op_dest;
 }
 
-std::optional<int64_t> CWallet::GetOldestKeyPoolTime() const
-{
-    LOCK(cs_wallet);
-    if (m_spk_managers.empty()) {
-        return std::nullopt;
-    }
-
-    std::optional<int64_t> oldest_key{std::numeric_limits<int64_t>::max()};
-    for (const auto& spk_man_pair : m_spk_managers) {
-        oldest_key = std::min(oldest_key, spk_man_pair.second->GetOldestKeyPoolTime());
-    }
-    return oldest_key;
-}
-
 void CWallet::MarkDestinationsDirty(const std::set<CTxDestination>& destinations) {
     for (auto& entry : mapWallet) {
         CWalletTx& wtx = entry.second;

src/wallet/wallet.h

@@ -733,8 +733,6 @@ public:
     size_t KeypoolCountExternalKeys() const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
     bool TopUpKeyPool(unsigned int kpSize = 0);
 
-    std::optional<int64_t> GetOldestKeyPoolTime() const;
-
     // Filter struct for 'ListAddrBookAddresses'
     struct AddrBookFilter {
         // Fetch addresses with the provided label