config(language): remove unmaintained languages

French, Greek, Polish and Spanish are not maintained for a
while now, there's actually no activity on Crowdin for those
languages.

30. January 2022:
French:  11% translated
Greek:    5% translated
Polish:  33% translated
Spanish: 51% translated

Languages can be added back if there're contributions visible
and at least 70% of Translation are reached.

German (90% translated) and Italian (73% translated) are still
available via config / admin panel.

An information was posted on Crowdin to Translators,
in our Telegram group and on GitHub (#64 (comment)).

Change-Id: I76dfedece946a223f5eca99d2ad90a10b7ad78aa

config/config.inc.php

@@ -7,7 +7,7 @@
 $config = array();
 
 // G E N E R A L
-// possible language values: de, el, en, es, fr, pl, it
+// possible language values: de, en, it
 $config['ui']['language'] = 'en';
 $config['adminpanel']['view'] = 'basic';
 $config['dev']['enabled'] = true;

lib/configsetup.inc.php

@@ -78,11 +78,7 @@
             'placeholder' => $defaultConfig['ui']['language'],
             'options' => [
                 'de' => 'DE',
-                'el' => 'EL',
                 'en' => 'EN',
-                'es' => 'ES',
-                'fr' => 'FR',
-                'pl' => 'PL',
                 'it' => 'IT',
             ],
             'value' => $config['ui']['language'],