Do not prune images not specified by user

Summary: It is typical for operator publishers to remove operator versions from package channels with new versions utilizing the skip(range) to queue upgrades from deleted versions. oc-mirror detects this condition as content removed from the imageset and prunes associated images from the deleted version from the mirror registry. This RFE proposes a scheme for overcoming this potentially unexpected behavior.

Fix proposal: https://github.com/openshift/oc-mirror/issues/462