It's in your profile folder.
Look at "About Opera" in the menu to see where the profile folder is.
Note that the file name is just "Bookmarks", it does not have a file name extension.
I would simply cut and paste it to the desktop temporarily (with Opera closed), run Opera and it should create a new blank file with no bookmarks.
If you're happy with that, then delete the first file from the desktop.
šŸ™‚