ENGIN-X: Remove drop-down list in stress rig OPI

As a user I don't want my control channel to be changed accidentally by scrolling the mouse wheel. This is possible at the minute because this setpoint is controlled by a drop-down list.

This is not a trivial change as the stress rig OPI cannot be loaded in the IBEX version of CSS. Some (but not all) of the OPI can be loaded in using the latest version of CSS off the web, but doing so loses some of the graphs. This issue needs to be addressed before the OPI can be changed.

### Acceptance criteria
- [ ] Control channel setpoint no longer changed using a drop down menu
- [ ] Stress rig OPI can be loaded/saved in CSS without losing elements