build(core): remove main() from specs (#21053)

`main()` function used to be needed to support dart, since dart
Does not allow top level statements. Since we no longer use dart
The need for `main()` has been removed.

In preparation for `Basel` and standardized way of running tests
we are removing `main()`

PR Close #21053

packages/core/test/linker/integration_spec.ts

@@ -28,7 +28,7 @@ import {stringify} from '../../src/util';
 
 const ANCHOR_ELEMENT = new InjectionToken('AnchorElement');
 
-export function main() {
+{
   describe('jit', () => { declareTests({useJit: true}); });
 
   describe('no jit', () => { declareTests({useJit: false}); });