Codechange: use "[[maybe_unused]]" instead of a wide variety of other ways we had

While at it, replace OTTD_ASSERT with WITH_ASSERT, as this
is always set if assert() is valid. No matter if NDEBUG is set
or not.

src/vehiclelist.cpp

@@ -53,8 +53,7 @@ bool VehicleListIdentifier::UnpackIfValid(uint32 data)
 /* static */ VehicleListIdentifier VehicleListIdentifier::UnPack(uint32 data)
 {
 	VehicleListIdentifier result;
-	bool ret = result.UnpackIfValid(data);
-	(void)ret; // assert only
+	[[maybe_unused]] bool ret = result.UnpackIfValid(data);
 	assert(ret);
 	return result;
 }