[a11y cleanup] Remove labelledBy ID

- as far as I can tell it's not doing anything meaningful on the popover, and the content was previously not predictable or useful

- native `select`s do not have an association with the select label and the dropdown, so I don't think this component needs to either

src/components/form/super_select/super_select.tsx

@@ -106,7 +106,6 @@ export class EuiSuperSelect<T extends string> extends Component<
   private _isMounted: boolean = false;
 
   describedById = htmlIdGenerator('euiSuperSelect_')('_screenreaderDescribeId');
-  labelledById = htmlIdGenerator('euiSuperSelect_')('_screenreaderLabelId');
 
   state = {
     isPopoverOpen: this.props.isOpen || false,
@@ -293,7 +292,6 @@ export class EuiSuperSelect<T extends string> extends Component<
 
     const button = (
       <EuiSuperSelectControl
-        screenReaderId={this.labelledById}
         options={options}
         value={valueOfSelected}
         placeholder={placeholder}
@@ -351,7 +349,6 @@ export class EuiSuperSelect<T extends string> extends Component<
           </p>
         </EuiScreenReaderOnly>
         <div
-          aria-labelledby={this.labelledById}
           aria-describedby={this.describedById}
           className="euiSuperSelect__listbox"
           role="listbox"

src/components/form/super_select/super_select_control.tsx

@@ -71,11 +71,6 @@ export interface EuiSuperSelectControlProps<T>
    * `string` | `ReactElement` or an array of these
    */
   append?: EuiFormControlLayoutProps['append'];
-  /**
-   * Creates a semantic label ID for the `div[role="listbox"]` that's opened on click or keypress.
-   * __Generated and passed down by `EuiSuperSelect`.__
-   */
-  screenReaderId?: string;
 }
 
 export const EuiSuperSelectControl: <T extends string>(
@@ -97,7 +92,6 @@ export const EuiSuperSelectControl: <T extends string>(
     placeholder,
     prepend,
     append,
-    screenReaderId,
     disabled,
     ...rest
   } = props;