Remove unused constant MEMPOOL_GD_VERSION

src/version.h

@@ -27,9 +27,6 @@ static const int CADDR_TIME_VERSION = 31402;
 //! BIP 0031, pong message, is enabled for all versions AFTER this one
 static const int BIP0031_VERSION = 60000;
 
-//! "mempool" command, enhanced "getdata" behavior starts with this version
-static const int MEMPOOL_GD_VERSION = 60002;
-
 //! "filter*" commands are disabled without NODE_BLOOM after and including this version
 static const int NO_BLOOM_VERSION = 70011;