Simple refactorings for OpsController::Settings::Common#settings_get_form_vars. Reduces flog from 1112 to 700 (~-37%).

 I can't test this so I don't know if it works, but it should be straight up algebraic substitutions for everything.

```
#params    263 ->   1
#[]        609 -> 442
assignment 161 -> 164
```
Because it reduces the number of calls to `params` from 263 to 1. Increases assignments by 1. Method calls will always be the most expensive thing in ruby. The extract-to-local refactoring here is perfect to reduce needless cost.