chore: fix lint error

Author: crisbeto

src/lib/core/observe-content/observe-content.ts

@@ -10,7 +10,6 @@ import {
   AfterContentInit,
   Injector,
 } from '@angular/core';
-import {Observable} from 'rxjs/Observable';
 import {Subject} from 'rxjs/Subject';
 import 'rxjs/add/operator/debounceTime';