refactor(core): remove unused export (#38224)

This export used to be here to turn this file into an ES Module - this is no longer needed
because the file contains imports.

PR Close #38224

packages/core/testing/src/before_each.ts

@@ -26,7 +26,3 @@ if (_global.beforeEach) {
     resetFakeAsyncZone();
   });
 }
-
-// TODO(juliemr): remove this, only used because we need to export something to have compilation
-// work.
-export const __core_private_testing_placeholder__ = '';