Change CMessageHeader Constructor

This commit removes the single-parameter contructor of CMessageHeader
and replaces it with a default constructor.

The single parameter contructor isn't used anywhere except for tests.
There is no reason to initialize a CMessageHeader with a particular
messagestart.  This messagestart should always be replaced when
deserializing an actual message header so that we can run checks on it.

The default constructor initializes it to zero, just like the command
and checksum.

This also removes a parameter of a V1TransportDeserializer constructor,
as it was only used for this purpose.

src/test/fuzz/deserialize.cpp

@@ -190,7 +190,7 @@ void test_one_input(const std::vector<uint8_t>& buffer)
         AssertEqualAfterSerializeDeserialize(s);
 #elif MESSAGEHEADER_DESERIALIZE
         const CMessageHeader::MessageStartChars pchMessageStart = {0x00, 0x00, 0x00, 0x00};
-        CMessageHeader mh(pchMessageStart);
+        CMessageHeader mh;
         DeserializeFromFuzzingInput(buffer, mh);
         (void)mh.IsValid(pchMessageStart);
 #elif ADDRESS_DESERIALIZE