user_post_update_update_migrated_roles_followup() needs to be a hook_update_N

We already have test coverage in \Drupal\Tests\user\Functional\Update\UserUpdateRoleMigrateTest - I'm not sure that we need to batch these updates like I doubt that there are sites with more than a few hundred roles and we shouldn't be scared of a couple of hundred of updates. And this would be a very very very extreme case.

Discussed with @catch we agreed to add a comment to user.install saying the D10 has a new update so adding one here is not recommended.

+++ b/core/modules/user/user.post_update.php
@@ -18,39 +14,3 @@ function user_removed_post_updates() {
-/**
- * Remove non-existent permissions created by migrations.
- */
-function user_post_update_update_migrated_roles_followup(&$sandbox = NULL) {

Do we need to add this to the list of removed updates? Or do we need to have it as an empty function? This has been released in a beta...

Discussed with @catch and @longwave. @longwave pointed out that we'd removed an update during a beta before - see #3145501: updb error processMultivalueBaseFieldHandler() - I think this is okay tbh and cleaner than leaving an empty stub that people are going to see when updating to D10.