xAODRootAccess: Fix cppyy nodiscard warnings.

cppyy will generate wrappers both for functions returning a value and the value being discarded. If we have [[nodiscard]] on, then we'll get warnings when cling compiles these wrappers. Since these wrappers are automatically generated by cppyy, we can't really avoid these other than by disabling them for the cling case.