code quality: fix usage of remove_query_arg() in wp-activate.php

remove_query_arg() only takes `string|string[]`. Added a comment because it took me 5 minutes to understand _why_ we were using

Author: justlevine

src/wp-activate.php

@@ -37,7 +37,8 @@
 if ( $key ) {
 	$redirect_url = remove_query_arg( 'key' );
 
-	if ( remove_query_arg( false ) !== $redirect_url ) {
+	// If we still have query args, stash the key and redirect to handle them.
+	if ( remove_query_arg( '' ) !== $redirect_url ) {
 		setcookie( $activate_cookie, $key, 0, $activate_path, COOKIE_DOMAIN, is_ssl(), true );
 		wp_safe_redirect( $redirect_url );
 		exit;