docs(material/expansion): remove unused styling (#28040)

this particular example had css for a mdc form field that isn't reference in the example

Author: shinkhouse

src/components-examples/material/expansion/expansion-overview/expansion-overview-example.css

@@ -7,7 +7,6 @@ import {MatExpansionModule} from '@angular/material/expansion';
 @Component({
   selector: 'expansion-overview-example',
   templateUrl: 'expansion-overview-example.html',
-  styleUrls: ['expansion-overview-example.css'],
   standalone: true,
   imports: [MatExpansionModule],
 })