Problem with get_page_by_path() using with WP_Query

It doesn’t make any sense to try and pass a query result to get_page_by_path(). The point of WP_Query is to query for a selection of posts based on given criteria, while get_page_by_path() is for retrieving a single specific post based on its path. There’s no reason you’d need to use them together.

If you want to delete posts based on their slug, then you don’t need a query at all:

$pages = array( 'member-login', 'member-account', 'member-register', 'member-password-lost', 'member-password-reset' );

foreach ( $pages as $slug ) {
    $page = get_page_by_path( $slug );

    if ( $page ) {
        wp_delete_post( $page->ID, true );
    }
}