strip: use bundle2 + cg2 by default when repository use general delta The bundle10 format (plain changegroup-01) does not support general delta and result into expensive delta re-computation when stripping. If the repository is general delta, we store backups as bundle20 containing a changegroup-02 payload. We remove the experimental feature related to strip backup bundle format because this achieve the same goal in a leaner way. Removing the experimental option is fine, that is why it experimental in the first place. Compression of these bundles are coming in later changesets.