Comment 2 for bug 1817961

Reviewed: https://review.openstack.org/639840
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=47061e699b9dbf6fdeea572a5abeaa72e499ec87
Submitter: Zuul
Branch: master

commit 47061e699b9dbf6fdeea572a5abeaa72e499ec87
Author: Matt Riedemann <email address hidden>
Date: Wed Feb 27 16:24:24 2019 -0500

    Optimize populate_queued_for_delete online data migration

    The data migration was needlessly querying the cell database
    for instances even if there were no instance mappings in that
    database that needed to be migrated. This simply continues to
    the next cell if the instance mappings in the current cell are
    migrated.

    While we're in here, the joinedload on 'cell_mapping' can be
    removed since it's not used.

    Closes-Bug: #1817961

    Change-Id: Idf35ed9d57945bc80fbd47393b7de076330160e6