Code Quality: Remove unused variable in wp-admin/includes/post.php.

Follow-up to [44241].

Props Soean, mukesh27.
Fixes #64871.

git-svn-id: https://develop.svn.wordpress.org/trunk@62040 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/post.php

@@ -2564,7 +2564,7 @@ function the_block_editor_meta_box_post_form_hidden_fields( $post ) {
 	$classic_output = ob_get_clean();
 
 	$classic_elements = wp_html_split( $classic_output );
-	$hidden_inputs    = '';
+
 	foreach ( $classic_elements as $element ) {
 		if ( ! str_starts_with( $element, '<input ' ) ) {
 			continue;