Call pantheon.php's pantheon_bulk_file_delete rather than drops-7 panthe...

Call pantheon.php's pantheon_bulk_file_delete rather than drops-7 pantheon_api_bulk_file_delete

Deleting whole file pantheon_api.inc since it only contains the function we are deprecating (pantheon_api_bulk_file_delete) and a function that it calls (pantheon_api_curl)