refacto(utils): remove a useless intermediate variable

This is the commit of the suggestion I made here:
https://framagit.org/framasoft/framadate/framadate/merge_requests/344/diffs#note_399810

app/classes/Framadate/Utils.php

@@ -34,15 +34,13 @@ class Utils {
             (isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] === 'https')
         ) ? 'https' : 'http';
 
+        $domain = defined('APP_URL') ? APP_URL : $serverName;
         $port = in_array($serverPort, ['80', '443'], true) ? '' : ':' . $serverPort;
         $dirname = dirname($_SERVER['SCRIPT_NAME']);
         $dirname = $dirname === '\\' ? '/' : $dirname . '/';
         $dirname = str_replace('/admin', '', $dirname);
         $dirname = str_replace('/action', '', $dirname);
 
-        $appUrlConfig = defined('APP_URL') ? APP_URL : '';
-        $domain = $appUrlConfig === '' ? $serverName : $appUrlConfig;
-        
         return $scheme . '://' . preg_replace('#//+#', '/', $domain . $port . $dirname);
     }