Delete old targets metadata in "snapshot()"

[jku]

• Don't delete previous targets version when a new one is made
• Instead delete target files when they are removed from snapshot meta

This allows two things:

• We keep number of metadata files to a reasonable figure (and remove any versions that are no longer reachable by current snapshot)
• The repository remains valid and usable between addition of new targets files and snapshot update.

Signed-off-by: Jussi Kukkonen jkukkonen@vmware.com

Fixes #2

[jku]

Some manual testing would be good before merging this...