Put back seq_to_removal_str() to private
Add mapper.remove_oid() to remove SnapMapper items
Fix finish_remove_pgs() to reflect the way load_pgs() works
Pass next_removal_seq as pointer since it is modified
Remove dead code
Use collection_exists() not list_collections() because there are no snap collections

This is still an intermediate implementation
Just adding --type remove instead of new command name.

Signed-off-by: David Zafman <david.zafman@inktank.com>