Remove test_export_es6_allows_export_in_post_js. NFC (#23240)

This test was added in #13937. Here we are testing something that we
actually decided we did not want to support. Specifically in #13894 we
decided that `--extern-post-js` was the place to allow this.

More importantly the resulting JS module simple doesn't work because in
`MODULARIZE` mode the code from `--post-js` does not end up at the top
level but instead inside a factory function. Note that this test does
not try to exectute the resulting JS. If you try to execute it you get
`SyntaxError: Unexpected token 'export'`.

Since we don't support it I think its best to remove this test.

I'm also working on a change to move the modularization code before
acorn and closure, and with that change this won't even compile (because
the modularization process puts all the code inside and inner scope).

Author: sbc100

test/export_module.js

@@ -425,11 +425,6 @@ def test_export_es6_node_requires_import_meta(self):
                             '-sENVIRONMENT=node', '-sEXPORT_ES6', '-sUSE_ES6_IMPORT_META=0'])
     self.assertContained('EXPORT_ES6 and ENVIRONMENT=*node* requires USE_ES6_IMPORT_META to be set', err)
 
-  def test_export_es6_allows_export_in_post_js(self):
-    self.run_process([EMCC, test_file('hello_world.c'), '-O3', '-sEXPORT_ES6', '--post-js', test_file('export_module.js')])
-    src = read_file('a.out.js')
-    self.assertContained('export{doNothing};', src)
-
   @parameterized({
     '': (False,),
     'package_json': (True,),