Erase writable label in Prepare if not overlaybd

[HileQAQ]

What this PR does / why we need it:
In buildkit build, we will add containerd.io/snapshot/overlaybd.writable=dev when --oci-worker-snapshotter=overlaybd. For compatibility, we should delete this label in Prepare for OCI image, or else it will lead to an error.
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?
• Do all new files have an appropriate license header?

[liulanzheng]

LGTM.
writable label for oci image should be removed