added delete volunteer task route

[KimLinhVu]

• added a delete route to Volunteer that deletes a task object in task table of Opportunity
• given an Opportunity Object Id (oppId) and taskId in the request body, removes the task that matches taskId from task table in opportunity object

[ryanchansf]

Right now, the second pass of filtering through the oppList does not function as intended due to a mismatch in taskIds. Because of this, we should filter out the objects by their startTime and endTime. In the first pass of filtering, when the task is found via its id, store its start and end times in startTime and endTime variables. Then, in the second through the oppList, we can filter out the task object again by making sure that both times match. Note that the start and end fields are both arrays — not sure why the schema was built this way, but make sure that this comparison is between the start and end values of the task object. Finally, test the route in postman for a given taskId and make sure the changes are as expected in the database. Thanks again!