Multisite: Handle capability check for removing oneself via `map_meta_cap()`.

Site administrators should not be able to remove themselves from a site. This moves the enforcement of this rule from `wp-admin/users.php` to `remove_user_from_blog()` via the `remove_user` capability, which furthermore allows us to get rid of two additional clauses and their `is_super_admin()` checks in `wp-admin/users.php`. A unit test for the new behavior has been added.

Fixes #39063. See #37616.

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