refactor(@angular-devkit/core): delete deepCopy temporary symbol property after use

The `deepCopy` utility function previously set its internal tracking symbol property to `undefined` after completion.
However, this still caused the property to exist with a value of `undefined`. Jasmine 4.1 now checks for symbols with
its `toEqual` expectation which resulted in failing tests. The internal tracking symbol property is now removed instead
via `delete`.

Author: clydin

packages/angular_devkit/core/src/utils/object.ts

@@ -33,7 +33,7 @@ export function deepCopy<T>(value: T): T {
     for (const key of Object.getOwnPropertyNames(valueCasted)) {
       copy[key] = deepCopy(valueCasted[key]);
     }
-    valueCasted[copySymbol] = undefined;
+    delete valueCasted[copySymbol];
 
     return copy;
   } else {