MDL-87708 admin: Remove tool_moodlenet plugin

Part of MDL-87351

The MoodleNet integration plugin has been removed from core
due to the shutdown of MoodleNet public service in April 2026.

Sites using self-hosted MoodleNet instances can install the
plugin from the Moodle Plugins Directory.

lib/plugins.json

@@ -497,7 +497,6 @@
             "mfa",
             "mobile",
             "monitor",
-            "moodlenet",
             "multilangupgrade",
             "oauth2",
             "phpunit",
@@ -702,6 +701,7 @@
             "bloglevelupgrade",
             "health",
             "innodb",
+            "moodlenet",
             "qeupgradehelper",
             "timezoneimport"
         ],