remove "checkorder" and "reply" P2P commands

These command are a leftover from send-to-IP transactions, which have been
removed a long time ago.
Also removes CNode::mapRequests and CNode::PushRequests, as these were
only used for the mentioned commands.

src/main.h

@@ -20,7 +20,6 @@ class CReserveKey;
 
 class CAddress;
 class CInv;
-class CRequestTracker;
 class CNode;
 
 static const unsigned int MAX_BLOCK_SIZE = 1000000;