genie_python: Remove dummy argument from load_script

[DominicOram]

As a user I would not like a confusing genie_python API, the load_script command currently contains a dummy parameter that does nothing but allow the GUI to call genie_python in a legacy way. the GUI should stop using this legacy code and the dummy parameter should be removed.

Acceptance Criteria

• The GUI should stop using the legacy load_script
• The dummy parameter should be removed

[DominicOram]

See ISISComputingGroup/ibex_gui#1119 and https://github.com/ISISComputingGroup/genie_python/pull/259