I am not a developer for noteself, so you should confirm all this on the forum.
If I remember correctly, if you don't set noteself use a remote database, it uses the in-browser indexDB to store wikis. This lives in the persistent browser cache, the wikis are not saved elsewhere unless you download a copy. So clearing all the data in the browser cache will remove any wikis saved.
There should be options to not delete persistent data when clearing the browser cache, but the process is almost certainly going to be different for each browser.