Make "Yes" button unfilled (#18960)

This changes the button from looking like it has been pre-clicked, which
is not the case.

x-pack/plugins/xpack_main/public/hacks/welcome_banner/opt_in_banner_component.js

@@ -102,7 +102,6 @@ export class OptInBanner extends Component {
           <EuiFlexItem grow={false}>
             <EuiButton
               size="s"
-              fill
               onClick={() => this.props.optInClick(true)}
             >
               Yes