Theme Switcher: Use the global panel back button instead of adding a new one.

This ensures the back button is visible on iOS devices. Same as in [29610].

see #31794.
Built from https://develop.svn.wordpress.org/trunk@31918


git-svn-id: http://core.svn.wordpress.org/trunk@31897 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-customize-section.php

@@ -345,7 +345,6 @@ class WP_Customize_Themes_Section extends WP_Customize_Section {
 				<?php echo esc_html( $this->title ); ?>
 				<span class="screen-reader-text"><?php _e( 'Press return or enter to expand' ); ?></span>
 			</h3>
-			<span class="control-panel-back themes-panel-back" tabindex="-1"><span class="screen-reader-text"><?php _e( 'Back' ); ?></span></span>
 			<div class="customize-themes-panel control-panel-content themes-php">
 				<h2><?php esc_html_e( 'Themes' ); ?>
 					<span class="title-count theme-count"><?php echo count( $this->controls ) - 1; ?></span>