refactor(material/sort): remove deprecated APIs for v12

Cleans up the deprecated APIs that were marked for removal in v12.

BREAKING CHANGES:
* `MatSortHeaderIntl.sortButtonLabel` has been removed.
* `MatSortHeaderHarness.getAriaLabel` has been removed.

Author: crisbeto

src/material/sort/sort-header-intl.ts

@@ -20,16 +20,8 @@ export class MatSortHeaderIntl {
    * components if the labels have changed after initialization.
    */
   readonly changes: Subject<void> = new Subject<void>();
-
-  /**
-   * ARIA label for the sorting button.
-   * @deprecated Not used anymore. To be removed.
-   * @breaking-change 8.0.0
-   */
-  sortButtonLabel = (id: string) => {
-    return `Change sorting for ${id}`;
-  }
 }
+
 /** @docs-private */
 export function MAT_SORT_HEADER_INTL_PROVIDER_FACTORY(parentIntl: MatSortHeaderIntl) {
   return parentIntl || new MatSortHeaderIntl();

src/material/sort/testing/sort-header-harness.ts

@@ -47,15 +47,6 @@ export class MatSortHeaderHarness extends ComponentHarness {
     return '';
   }
 
-  /**
-   * Gets the aria-label of the sort header.
-   * @deprecated The sort header no longer has an `aria-label`. This method will be removed.
-   * @breaking-change 11.0.0
-   */
-  async getAriaLabel(): Promise<string|null> {
-    return (await this._container()).getAttribute('aria-label');
-  }
-
   /** Gets whether the sort header is currently being sorted by. */
   async isActive(): Promise<boolean> {
     return !!(await this.getSortDirection());

tools/public_api_guard/material/sort/testing.d.ts

@@ -7,7 +7,6 @@ export declare class MatSortHarness extends ComponentHarness {
 
 export declare class MatSortHeaderHarness extends ComponentHarness {
     click(): Promise<void>;
-    getAriaLabel(): Promise<string | null>;
     getLabel(): Promise<string>;
     getSortDirection(): Promise<SortDirection>;
     isActive(): Promise<boolean>;