It would not be an easy process, and you’d need to work directly with the db.
The first step would be to search the meta_value column of the db in the _termmeta, _postmeta, _usermeta and _commentmeta tables looking for any values that starts with “field_%”. You would also need to search for the same thing in the option_value column of the _options table. These are the field key references.
The meta_key value will look something like “_some_field_name”. If you remove the initial underscore you will have the meta_key where the actual field value is stored.
All of these would need to be deleted.
