fix: remove redundant form-control class for MasqueradeUserInput

As the FormControl element already comes with the form-control
class, the extra class specified on the MasqueradeUserInput is
applied to the div container of the <input> causing double input
box effect. This is not visible in themes where the container
div's padding is zero. But appears when padding is not set.

Internal-ref: https://tasks.opencraft.com/browse/BB-8929

src/instructor-toolbar/masquerade-widget/MasqueradeWidget.jsx

@@ -135,7 +135,7 @@ class MasqueradeWidget extends Component {
             <span className="col-auto col-form-label pl-3" id="masquerade-search-label">{`${specificLearnerInputText}:`}</span>
             <MasqueradeUserNameInput
               id="masquerade-search"
-              className="col-4 form-control"
+              className="col-4"
               autoFocus={autoFocus}
               defaultValue={masqueradeUsername}
               onError={(errorMessage) => this.onError(errorMessage)}