I wanted to create a WP plugin. So I requested and my request was approved by WordPress.org But I don’t want to develop that plugin anymore. How can I delete the repository forever or request its deletion.

Please mail plugins@wordpress.org
and ask them to remove your plugin. Provide all infos and explain that you won’t start to develop the plugin so the name should no longer be blocked.
I would use the same email as while registering the plugin (maybe even replying to it) and I think they will remove it after some days.