Spec Lite: Remove sqlalchemy-migrate

project: glance
problem: The sqlalchemy-migrate library is unmaintained and has been deprecated in favour of alembic. It is not compatible with sqlalchemy 2.0 and is issuing many warnings suggesting Python 3.10 compatibility may be in doubt. Glance switched to alembic for new migrations during the Ocata cycle and no longer needs to carry the old sqlalchemy-migrate-based migrations.
solution: Remove the sqlalchemy-migrate-based migrations and related tooling.
impacts: None. Users coming from a pre-Ocata deployment will need to deploy a pre-Xena glance before deploying Xena, but that's standard practice anyway.