Script Generator: Remove sgp file

As a user I am confused by having two files associated with the script generator. In a discussion between @JamesKingWork and I (from https://github.com/ISISComputingGroup/IBEX/issues/6670) we decided that the confusion over the multiple files is a significantly larger barrier to users than the problems it solves. We also decided that the JSON should be stored in plain text JSON in the generated file and runscript will load the values from here.

### Acceptance Criteria
* The parameters are written to the `value` variable in generated files in plain text JSON
* `runscript` uses this JSON to get the parameters to load
* The GUI loads parameters from generated scripts by looking at the value field and extracting the json
* The sgp file is removed