Instead of deleting one by you can either use server actions or create a python script for deleting all the invoices with attachments
Otherwise simply, enable developer mode and go to General Settings->Technical Menu->Attachments
Here you can see all the attachments, filter for the attachments related to the account.move model only (Resource Model is equal to "account.move")
In the list view select all and from the Actions you can click delete button and delete.
Please check the screenshots