obsstore: drop 'date' from the metadata dictionary
We are extracting the `date` information from the metadata at read time.
However, we failed to remove it from the metadata returned in the markers. This
is now fixed.
{header}
# HG changeset patch
# User {author}
# Date {date|hgdate}
# Node ID {node}
{parent%changesetparent}
{desc}
{diff}