prevent duplicate rendering of css class in form checkbox plugin

the provided `cssClass` is added in the `->getStyleClass()` method on the previous line.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | na
| Fixed tickets | 
| Refs tickets  | 
| License       | MIT
| Doc PR        |