Leech's Safari extension is part of the app itself, so it cannot be uninstalled on its own. To completely remove the extension, you need to delete Leech itself.

However, inactive extensions in Safari do absolutely nothing and require no resources. So if you'd like to keep Leech around but not use the extension, do the following:

Open Safari's Settings (Safari menu → Settings), then click on Extensions. Find "Leech Extension" in the list, and uncheck its "enabled" box. When done, it should look like this:

Once disabled, the Leech extension won't have any affect in Safari.