It is not possible to remove synced sheets & files from your Google Drive folder.
What happens if I delete a Google Sheet?
If you delete a synced Google sheet from your Google Drive folder, our app will automatically create a new Google sheets as soon as records were added or updated to the related data set.
Example: If you delete a Google Sheet of an Asana project, then a new Google Sheet will be created as soon as a task has been added or updated in that project.
What happens if I delete an attachment?
Attachments are only synced once from your data bacups. So if you a delete an attachment from your Google Drive, this attachment will not be synced again.