Fix behaviour of DeleteItem.

The current behaviour makes DeleteItem behave like UpdateItem
using the "Delete" action. This actually does not delete an item
but an attribute.

See http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_UpdateItem.html

This patch makes DeleteItem deleting items using the DeleteItem api
method.

See http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_DeleteItem.html