It won't go away on its own.
You can delete the file while Opera is closed.
Or, going to the URL opera://settings/clearBrowserData, switching to "advanced", selecting only "cookies and site data", setting the time range to "all time" and clicking "clear data" will probably get rid of it (but will clear that for all sites).
Instead, goto the URL opera://settings/content/all, sort by "data stored" and trash all entries for the site or trash the specific subentry that is using the 20GB.
Or, while on the site, hit ctrl + shift + i to open the developer tools, switch to the application tab in the developer tools, select "Storage" in the left-hand pane and then clear storage for the site in the right-hand pane.
