Unnecessary setting of `parameters["in_file"]` in `UiLayer._setup_loader()`

In the following code:
- the `parameters["in_file"]` gets set to `self.in_data_file`
- then `in_file` gets set to `parameters["in_file"]`

https://github.com/DiamondLightSource/httomo/blob/74b94c9140945211657617c00b7ff9694455d60d/httomo/ui_layer.py#L115-L118

and in the rest of the function, there's no further use of `parameters["in_file"]`.

It seems like defining the `"in_file"` key in the `parameters` dict isn't doing anything useful, the `parameters` dict is never used to get that key-value pair, so it feels like this is unnecessarily passing around information.