Remove unused `_get_discard_data_out()` function from YAML template generator

Now that we no longer have the `data_out` parameter in methods, we no longer need to do anything special for the case when the image saver didn't have a `data_out` parameter. Namely, we can remove this now unused function: https://github.com/DiamondLightSource/httomo/blob/878c74c32dad626b46a52f1b4a8505747f5fd978/templates/yaml_templates_generator.py#L163-L169

and remove its single (unused) usage in the YAML generator function: https://github.com/DiamondLightSource/httomo/blob/878c74c32dad626b46a52f1b4a8505747f5fd978/templates/yaml_templates_generator.py#L48