Make DELETE request return 404 for nonexistant package

Change the package_delete function to use the _package_get helper
function which will correctly raise the right exception for a
package not existing.

Change-Id: I1e83067b9769221dc5531d2de891960ce7846b6e
Closes-Bug: #1309413
Merge "Make DELETE request return 404 for nonexistant package"