REST API: Remove 'Invalid user ID' error in WP_REST_Users_Controller::update_item().

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.
Built from https://develop.svn.wordpress.org/trunk@55325


git-svn-id: http://core.svn.wordpress.org/trunk@54858 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php

@@ -717,14 +717,6 @@ class WP_REST_Users_Controller extends WP_REST_Controller {
 
 		$id = $user->ID;
 
-		if ( ! $user ) {
-			return new WP_Error(
-				'rest_user_invalid_id',
-				__( 'Invalid user ID.' ),
-				array( 'status' => 404 )
-			);
-		}
-
 		$owner_id = false;
 		if ( is_string( $request['email'] ) ) {
 			$owner_id = email_exists( $request['email'] );

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.2-beta1-55324';
+$wp_version = '6.2-beta1-55325';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.