Removes the `WP_Error` code for `'Invalid user ID.'`. Why?
tl;dr
This branch will never be entered as the `$user` will never be falsey.
Longer reasoning:
[39954] introduced `WP_REST_Users_Controller::get_user()` method to encapsulate getting the user and handling the `'Invalid user ID.'` `WP_Error`. It replaced `get_userdata()` in `WP_REST_Users_Controller::update_item()` but left the existing `'Invalid user ID.'` `WP_Error` introduced in [38832].
The code removed in this changeset will never be reached because `$user` will never be falsey. Rather, `WP_REST_Users_Controller::get_user()` will always return an instance of `WP_Error` or `WP_User`.
Could the user's ID be falsey?
No. Why? `WP_REST_Users_Controller::get_user()` checks that the user exists, which checks if the ID is falsey.
Therefore, the code can safely be removed.
Follow-up to [39954], [38832].
Props jrf, costdev, hellofromTonya, SergeyBiryukov.
Fixes#56662.
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git-svn-id: http://core.svn.wordpress.org/trunk@54858 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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