Make GetSerializeSize a wrapper on top of CSizeComputer

Given that in default GetSerializeSize implementations created by
ADD_SERIALIZE_METHODS we're already using CSizeComputer(), get rid
of the specialized GetSerializeSize methods everywhere, and just use
CSizeComputer. This removes a lot of code which isn't actually used
anywhere.

For CCompactSize and CVarInt this actually removes a more efficient
size computing algorithm, which is brought back in a later commit.

src/primitives/transaction.h

@@ -281,7 +281,7 @@ public:
         if (scriptPubKey.IsUnspendable())
             return 0;
 
-        size_t nSize = GetSerializeSize(SER_DISK,0)+148u;
+        size_t nSize = GetSerializeSize(*this, SER_DISK, 0) + 148u;
         return 3*minRelayTxFee.GetFee(nSize);
     }