[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/init.cpp

@@ -74,6 +74,7 @@
 #include <boost/algorithm/string/classification.hpp>
 #include <boost/algorithm/string/replace.hpp>
 #include <boost/algorithm/string/split.hpp>
+#include <boost/signals2/signal.hpp>
 #include <boost/thread.hpp>
 
 #if ENABLE_ZMQ

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();

src/txmempool.cpp

@@ -355,7 +355,6 @@ void CTxMemPool::AddTransactionsUpdated(unsigned int n)
 
 void CTxMemPool::addUnchecked(const CTxMemPoolEntry &entry, setEntries &setAncestors, bool validFeeEstimate)
 {
-    NotifyEntryAdded(entry.GetSharedTx());
     // Add to memory pool without checking anything.
     // Used by AcceptToMemoryPool(), which DOES do
     // all the appropriate checks.
@@ -406,14 +405,12 @@ void CTxMemPool::addUnchecked(const CTxMemPoolEntry &entry, setEntries &setAnces
 
 void CTxMemPool::removeUnchecked(txiter it, MemPoolRemovalReason reason)
 {
-    CTransactionRef ptx = it->GetSharedTx();
-    NotifyEntryRemoved(ptx, reason);
     if (reason != MemPoolRemovalReason::BLOCK) {
         // Notify clients that a transaction has been removed from the mempool
         // for any reason except being included in a block. Clients interested
         // in transactions included in blocks can subscribe to the BlockConnected
         // notification.
-        GetMainSignals().TransactionRemovedFromMempool(ptx);
+        GetMainSignals().TransactionRemovedFromMempool(it->GetSharedTx());
     }
 
     const uint256 hash = it->GetTx().GetHash();

src/txmempool.h

@@ -27,7 +27,6 @@
 #include <boost/multi_index/hashed_index.hpp>
 #include <boost/multi_index/ordered_index.hpp>
 #include <boost/multi_index/sequenced_index.hpp>
-#include <boost/signals2/signal.hpp>
 
 class CBlockIndex;
 extern RecursiveMutex cs_main;
@@ -699,9 +698,6 @@ public:
 
     size_t DynamicMemoryUsage() const;
 
-    boost::signals2::signal<void (CTransactionRef)> NotifyEntryAdded;
-    boost::signals2::signal<void (CTransactionRef, MemPoolRemovalReason)> NotifyEntryRemoved;
-
 private:
     /** UpdateForDescendants is used by UpdateTransactionsFromBlock to update
      *  the descendants for a single transaction that has been added to the