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/test/hash_tests.cpp

@@ -122,7 +122,7 @@ BOOST_AUTO_TEST_CASE(siphash)
                      (uint64_t(x+4)<<32)|(uint64_t(x+5)<<40)|(uint64_t(x+6)<<48)|(uint64_t(x+7)<<56));
     }
 
-    CHashWriter ss(SER_DISK, CLIENT_VERSION);
+    CHashWriter ss{CLIENT_VERSION};
     CMutableTransaction tx;
     // Note these tests were originally written with tx.nVersion=1
     // and the test would be affected by default tx version bumps if not fixed.