Don't Require --instance-id or --instance-name to delete AMIs

[josh-padnick]

A few users have reported interest in using ec2-snapper to delete all AMIs older than a certain date, but without concern to which EC2 Instance they were created for, or whether those AMIs were originally created with ec2-snapper.

It'd be nice not to require the --instance-id or --instance-name features so that ec2-snapper could be used for this purpose. This feels like a straigtforward change. PRs welcome!