fix(material/stepper): remove deprecated animation definitions

Deletes publicly-exported animation definitions that were marked for removal in v21.

BREAKING CHANGE:
* `matStepperAnimations` symbol has been removed.

Author: crisbeto

goldens/material/stepper/index.api.md

@@ -151,12 +151,6 @@ export class MatStepper extends CdkStepper implements AfterViewInit, AfterConten
     static ɵfac: i0.ɵɵFactoryDeclaration<MatStepper, never>;
 }
 
-// @public @deprecated
-export const matStepperAnimations: {
-    readonly horizontalStepTransition: any;
-    readonly verticalStepTransition: any;
-};
-
 // @public
 export class MatStepperIcon {
     constructor(...args: unknown[]);

src/material/stepper/BUILD.bazel

@@ -88,7 +88,6 @@ ng_project(
         "step-header.ts",
         "step-label.ts",
         "stepper.ts",
-        "stepper-animations.ts",
         "stepper-button.ts",
         "stepper-icon.ts",
         "stepper-intl.ts",

src/material/stepper/public-api.ts

@@ -13,6 +13,5 @@ export * from './stepper';
 export * from './stepper-button';
 export * from './step-header';
 export * from './stepper-intl';
-export {matStepperAnimations} from './stepper-animations';
 export * from './stepper-icon';
 export * from './step-content';