obsolete: properly drop 'date' from metadata Mistakes were made while resolving rebase conflicts in 4bc96685a40c. This led to 'date' being preserved in metadata when reading markers from a binary stream. As a result, some known markers were seen as "new" when pulling. I noticed it because a no-op pulls from main added about 600 duplicated markers to my obsstore (for each pull). I do not believe we need to perform any specific action to actively de-duplicates existing obsstore. After this fix, duplicated markers will no be propagated and the few affected repositories can probably deal with duplication (or people can repull the obsstore from a clone). As a side effect, we decode metadata only once, reducing the impact of the hack in fm0 to store extra important data (parents and date).