Remove all @deprecated code for prepareLegacyRequest form core/lib/Drupal/Core/ and from core/tests/Drupal

Thanks @Hardik_Patel_12 for the patch. RTBC done.

+++ b/core/lib/Drupal/Core/Test/FunctionalTestSetupTrait.php
@@ -498,9 +498,6 @@ protected function rebuildAll() {
     // Explicitly call register() again on the container registered in \Drupal.
-    // @todo This should already be called through
-    //   DrupalKernel::prepareLegacyRequest() -> DrupalKernel::boot() but that
-    //   appears to be calling a different container.

Let's try doing the @todo here now this code flow has obviously changed and try removing the call to
$this->container->get('stream_wrapper_manager')->register(); as well.

If that doesn't work out let's change the @todo here to be:

    // @todo This should already be called through DrupalKernel::boot() but that
    //   appears to be calling a different container.

That said I think it will work out because in \Drupal\Core\Test\FunctionalTestSetupTrait::resetAll() we do $this->container = \Drupal::getContainer();

Seemingly unrelated fail, requeuing

So the fail is stream wrapper related; the test generates an image derivative and then modifies the image style, and then fails where the image derivative should have been deleted but wasn't.

I guess debugging this here is out of scope so we should put the reregistration and the @todo back in.

This was done as part of a wider issue in #3104307: Remove BC layers in various Drupal\Core components