Remove CHashWriter type

The type is only ever set, but never read via GetType(), so remove it.
Also, remove SerializeHash to avoid silent merge conflicts and use the
already existing GetHash() boilerplate consistently.

src/primitives/block.cpp

@@ -10,7 +10,7 @@
 
 uint256 CBlockHeader::GetHash() const
 {
-    return SerializeHash(*this);
+    return (CHashWriter{PROTOCOL_VERSION} << *this).GetHash();
 }
 
 std::string CBlock::ToString() const