Remove the User last_connect

[data-sync-user]

Once the Bigtable migration is completed we can remove the legacy last_connect field from the User.

This was previously written to DynamoDB, intended to be used as a secondary index for a cleanup script – – but was never utilized. It’s not written to Bigtable, so it and related things (such as the set_last_connect method can be removed completely).

┆Issue is synchronized with this Jira Task