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
```
You could, but the idea behind putting them above the entire `case` is to eventually extract each `when` body to its own method. You don't want to pass down `params` and `@edit` and everything else. You want to pass down what they actually need so the code more accurately expresses intent (and is smaller, cheaper, more readable, more testable, etc).