Remove tvin

This was a special feature for a project I was working on long ago.
It provided implicit behavior for trivial value (0 or "") which were
supposed to be interpreted as NULL.
They led to `operator==` potentially being serialized as 'IS NULL'.

It makes more sense to introduce explicit helpers, e.g.

 - equal_or_is_null(col, optional)
 - not_equal_or_is_not_null(col, optional)

tests/Prepared.cpp

@@ -108,7 +108,6 @@ int Prepared(int, char* [])
                                              t.gamma != parameter(t.gamma));
     auto p = db.prepare(s);
     p.params.alpha = 7;
-    p.params.alpha = sqlpp::tvin(0);
     using S = decltype(s);
     using P = sqlpp::make_parameter_list_t<S>;
     P npl;