[At_Libitum]

I assume you already started with the cleanup, so re-importing is likely not an option, otherwise an option exists for adding books (see preferences) but here you would need to modify the template used to extract the info from the filename (thus allowing you to ignore the [....] bits)

Another option, without the need for re-importing is the (fairly) new Search/Replace option in the Metadata Editor (tab appears if you select multiple books). It allows regexp searching and replacing in fields. Do read the warnings though. I would first create a dummy custom column and use that as target for the replacement. If everything goes as expected use that column as source to update the original title column.