wallet2: remove unused pool tx removed callback

on_pool_tx_removed has no implementation beyond the empty default callback
and is not wired through the wallet API layer.

The only call site passed txid after erasing the payment entry that owned it,
leaving a dangling reference.

src/wallet/wallet2.cpp

@@ -3596,8 +3596,6 @@ void wallet2::remove_obsolete_pool_txs(const std::vector<crypto::hash> &tx_hashe
     {
       MDEBUG("Removing " << txid << " from unconfirmed payments");
       m_unconfirmed_payments.erase(pit);
-      if (0 != m_callback)
-        m_callback->on_pool_tx_removed(txid);
     }
   }
 }

src/wallet/wallet2.h

@@ -136,8 +136,6 @@ private:
     virtual boost::optional<epee::wipeable_string> on_device_pin_request() { return boost::none; }
     virtual boost::optional<epee::wipeable_string> on_device_passphrase_request(bool & on_device) { on_device = true; return boost::none; }
     virtual void on_device_progress(const hw::device_progress& event) {};
-    // Common callbacks
-    virtual void on_pool_tx_removed(const crypto::hash &txid) {}
     virtual ~i_wallet2_callback() {}
   };