Use --noinput convention for delete_orphaned_plugins command

delete_orphaned_plugins is a destructive command.  So by default report
how many things would be deleted and then ask for confirmation before
deleting them.  But if --noinput argument is used then don't ask for
confirmation and just go ahead.

Fixes issue #1591
Merge pull request #1819 from foobacca/delete_orphaned_noinput

Use --noinput convention for delete_orphaned_plugins command