Code to remove landing pages from site search results. Assumes that templates are in the templates directory inside the theme.

functions.php

<?php

add_action( 'pre_get_posts', 'leaven_search_hide_landing_page' );
/**
 * Remove landing pages from the search results.
 *
 * @param $query \WP_Query
 *
 * @return mixed
 */
function leaven_search_hide_landing_page( $query ) {
	if ( $query->is_search && $query->is_main_query() && ! is_admin() ) {
		$query->set( 'meta_query', array(
			'relation' => 'OR',
			array(
				'key'     => '_wp_page_template',
				'compare' => 'NOT EXISTS',
			),
			array(
				'key'     => '_wp_page_template',
				'value'   => 'templates/page_landing.php',
				'compare' => '!=',
			),
		) );
	}

	return $query;
}