refactor: use isObservable provided by rxjs 6.1+ (#27668)

Refactor common, core, forms, router to use the isObservable method from rxjs 6.1+. Remove the isObservable method from core.

PR Close #27668

integration/side-effects/snapshots/common/esm2015.js

@@ -1 +1,3 @@
 import "@angular/core";
+
+import "rxjs";

integration/side-effects/snapshots/common/esm5.js

@@ -1,3 +1,5 @@
 import "tslib";
 
 import "@angular/core";
+
+import "rxjs";