Allow removal of entries from the project coordinator's Log Hours UI

Provide the ability for volunteer coordinators to remove entries on the CiviVolunteer Log Hours interface. This is understood to mean that, should a volunteer coordinator add an entry and wish to remove it before saving the batch of hours, a button or other interface element will enable her to do so.

The approach will be to review the JS logic involved in the "add another row" behavior and extend or reverse-engineer it as appropriate.