[Spaces] spaces selector page background (#199981)

Removed `panelled` property that was covering the background image.
From [EUI
docs](https://eui.elastic.co/#/templates/page-template/examples#simple-page-with-header-and-sections)
`panelled` - adds a white background and shadow to define the area.

__Closes: https://github.com/elastic/kibana/issues/194193__

## Release Note
Removed `panelled` property that was covering the background image for
Spaces Selector page.

x-pack/plugins/spaces/public/space_selector/__snapshots__/space_selector.test.tsx.snap

@@ -4,7 +4,6 @@ exports[`it renders with custom logo 1`] = `
 <_KibanaPageTemplate
   className="spcSpaceSelector"
   data-test-subj="kibanaSpaceSelector"
-  panelled={true}
 >
   <EuiPortal>
     <div
@@ -65,7 +64,6 @@ exports[`it renders without crashing 1`] = `
 <_KibanaPageTemplate
   className="spcSpaceSelector"
   data-test-subj="kibanaSpaceSelector"
-  panelled={true}
 >
   <EuiPortal>
     <div

x-pack/plugins/spaces/public/space_selector/space_selector.tsx

@@ -117,11 +117,7 @@ export class SpaceSelector extends Component<Props, State> {
     }
 
     return (
-      <KibanaPageTemplate
-        className="spcSpaceSelector"
-        data-test-subj="kibanaSpaceSelector"
-        panelled
-      >
+      <KibanaPageTemplate className="spcSpaceSelector" data-test-subj="kibanaSpaceSelector">
         {/* Portal the fixed background graphic so it doesn't affect page positioning or overlap on top of global banners */}
         <EuiPortal>
           <div