MDL-71473 lib: Remove XMPPHP (Jabber) third-party lib

As the message_jabber notification plugin has been removed from core,
the XMPPHP library (aka Jabber) is not required any more so it has
been completely removed from Moodle core too.

lib/classes/component.php

@@ -96,7 +96,6 @@ class core_component {
         'MoodleHQ\\RTLCSS' => 'lib/rtlcss',
         'ScssPhp\\ScssPhp' => 'lib/scssphp',
         'Box\\Spout' => 'lib/spout/src/Spout',
-        'BirknerAlex\\XMPPHP' => 'lib/jabber/XMPP',
         'MatthiasMullie\\Minify' => 'lib/minify/matthiasmullie-minify/src/',
         'MatthiasMullie\\PathConverter' => 'lib/minify/matthiasmullie-pathconverter/src/',
         'IMSGlobal\LTI' => 'lib/ltiprovider/src',