refactor(animations): remove publicApi annotations from triggers (#42763)

the buildTrigger function and AnimationTrigger class are annotated
as publicApi which seems wrong and makes changes to the two
problematic, so they are being removed here

see: https://github.com/angular/angular/pull/42763/files#r671481902

PR Close #42763

packages/animations/browser/src/dsl/animation_trigger.ts

@@ -15,17 +15,11 @@ import {AnimationStyleNormalizer} from './style_normalization/animation_style_no
 
 
 
-/**
- * @publicApi
- */
 export function buildTrigger(
     name: string, ast: TriggerAst, normalizer: AnimationStyleNormalizer): AnimationTrigger {
   return new AnimationTrigger(name, ast, normalizer);
 }
 
-/**
- * @publicApi
- */
 export class AnimationTrigger {
   public transitionFactories: AnimationTransitionFactory[] = [];
   public fallbackTransition: AnimationTransitionFactory;