[mempool] Remove NotifyEntryAdded and NotifyEntryRemoved callbacks

NotifyEntryAdded never had any subscribers so can be removed.

Since ConnectTrace no longer subscribes to NotifyEntryRemoved, there are
now no subscribers.

The CValidationInterface TransactionAddedToMempool and
TransactionRemovedFromMempool methods can now provide this
functionality. There's no need for a special notifications framework for
the mempool.

src/interfaces/node.cpp

@@ -37,6 +37,8 @@
 
 #include <univalue.h>
 
+#include <boost/signals2/signal.hpp>
+
 class CWallet;
 fs::path GetWalletDir();
 std::vector<fs::path> ListWalletDir();