Update or Remove GUI related files

The [GUI](https://github.com/RSE-Sheffield/REMO/blob/main/src/run_gui.py) does not appear to be up to date. Are we planning to include it as part of this work? If so, could it be updated accordingly? If not, can we remove all related [files](https://github.com/RSE-Sheffield/REMO/tree/main/src/remo/gui)?