How to uninstall?

First steps

1. At admin area Disable Lightning and Clear Caches (if this is possible)
2. Remove OpenCart Lightning Modification and Refresh Modifications or - for OpenCart 1.5: delete vqmod/xml/lightning.xml file
3. Delete catalog/controller/extension/lightning folder

That is enough to uninstall Lightning. If you want to make totally everything back, you can take

Additional steps

1. Delete system/storage/cache/lightning folder
2. Delete admin/view/.htaccess, catalog/view/.htaccess and image/.htaccess files
3. Delete oc_lightning_modified and oc_lightning_product_to_page database tables, if they exist.
4. Delete this line from the root index.php:

if (file_exists($li = DIR_APPLICATION.'/controller/extension/lightning/gamma.php')) require_once($li); //Lightning