Remove SER_GETHASH, hard-code client version in CKeyPool serialize

It was never set, so it can be removed along with any code reading it.

src/serialize.h

@@ -131,7 +131,6 @@ enum
     // primary actions
     SER_NETWORK         = (1 << 0),
     SER_DISK            = (1 << 1),
-    SER_GETHASH         = (1 << 2),
 };
 
 /**

src/wallet/scriptpubkeyman.h

@@ -123,20 +123,14 @@ public:
     template<typename Stream>
     void Serialize(Stream& s) const
     {
-        int nVersion = s.GetVersion();
-        if (!(s.GetType() & SER_GETHASH)) {
-            s << nVersion;
-        }
+        s << int{259900}; // Unused field, writes the highest client version ever written
         s << nTime << vchPubKey << fInternal << m_pre_split;
     }
 
     template<typename Stream>
     void Unserialize(Stream& s)
     {
-        int nVersion = s.GetVersion();
-        if (!(s.GetType() & SER_GETHASH)) {
-            s >> nVersion;
-        }
+        s >> int{}; // Discard unused field
         s >> nTime >> vchPubKey;
         try {
             s >> fInternal;