When working on a task and you were in the need to delete a group of subtasks, you just simply selected the wanted subtasks to delete and then do the bulk action of click in the option to “Delete selected subtasks”:
Now it looks like Asana remove this option from the action list for a task and we can ony “Delete task”, and if you want to delete a subtask is a similar process (not able to do in bulk anymore).
I have many clients already complaining about this so here is what I have teach them:
1.Use the list view of the project (always my best recommendation, the board view is very limited in these kind of actions) and expand the subtasks.
2.Select the wanted subtasks to delete and click the 
trash bin icon from the floating menu at the button of your Asana screen to delete the subtasks.
It is for sure not effective, specially if you are dealing with a project with multiple tasks, but many clients use the subtasks as a checklist to ensure the SOPs are followed correctly, and from time to time there are cases where something just needs to be deleted.
Bulk actions are needed for big projects and it makes no sense having to go back to individual actions.
PS: Here is a scribe guide in case you want to save it: How to Bulk Delete Subtasks in Asana.