refactor(observers): remove 6.0.0 deletion targets (#10267)

Removes the deletion targets for 6.0.0 in the `cdk/observers` entry point.

BREAKING CHANGES:
* `ObserveContent` which was deprecated in 5.0.0 has been removed. Use `CdkObserveContent` instead.

src/cdk/observers/public-api.ts

@@ -7,9 +7,3 @@
  */
 
 export * from './observe-content';
-
-/**
- * @deprecated Use CdkObserveContent
- * @deletion-target 6.0.0
- */
-export {CdkObserveContent as ObserveContent} from './observe-content';