fixes for usage and seal

[liulanzheng]

What this PR does / why we need it:

• remove index after seal Remove writable index for overlaybd writable layer after a successful seal to save storage space.
• fix usage calculation for overlaybd snapshot Some space occupancy of overlaybd is not being counted, for example:
  • the lazy downloaded blobs that are not counted for the committed snapshots
  • writable layer space usages are not counted for active snapshots

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #

Please check the following list:

• [ ] Does the affected code have corresponding tests, e.g. unit test, E2E test?
• [ ] Does this change require a documentation update?
• [ ] Does this introduce breaking changes that would require an announcement or bumping the major version?
• [x] Do all new files have an appropriate license header?

[liulanzheng]

@WaberZhuang please check whether the usage fix is also worked for turbo-oci