Administration: Remove empty form action attributes.

Remove the `action` attribute in the login language selector, privacy forms, and classic widget forms.

An empty `action` attribute is invalid HTML4 and unsupported HTML5. The `action` attribute is optional, but must have a valid URL when provided.

Props Malae, audrasjb, bartkleinreesink, nicolefurlan, shubhamsedani, costdev, peterwilsoncc, rajinsharwar, joedolson.
Fixes #58226.

git-svn-id: https://develop.svn.wordpress.org/trunk@57295 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/widgets-form.php

@@ -478,7 +478,7 @@ foreach ( $wp_registered_sidebars as $sidebar => $registered_sidebar ) {
 
 				<?php if ( $is_inactive_widgets ) { ?>
 				<div class="remove-inactive-widgets">
-					<form action="" method="post">
+					<form method="post">
 						<p>
 							<?php
 							$attributes = array( 'id' => 'inactive-widgets-control-remove' );