How do you delete a content type's URL aliases in bulk?
I'd do it as an SQL query. Run as a SELECT first, to verify that all is well:

 SELECT alias FROM node n JOIN url_alias u ON CONCAT('node/', n.nid) =
 u.source WHERE n.type='my_type'


then edit it to make it a DELETE:

 DELETE u  FROM node n JOIN url_alias u ON CONCAT('node/', n.nid) =
 u.source WHERE n.type='my_type'