Don't run pre-migration checks for downgrade (#23634)

These checks are only make sense for upgrades.  Generally they exist to resolve referential integrity issues etc before adding constraints.  In the downgrade context, we generally only remove constraints, so it's a non-issue.

(cherry picked from commit 9ab9cd4)

airflow/utils/db.py

@@ -1504,16 +1504,6 @@ def downgrade(*, to_revision, from_revision=None, show_sql_only=False, session:
     log.info("Attempting downgrade to revision %s", to_revision)
     config = _get_alembic_config()
 
-    errors_seen = False
-    for err in _check_migration_errors(session=session):
-        if not errors_seen:
-            log.error("Automatic migration failed.  You may need to apply downgrades manually.  ")
-            errors_seen = True
-        log.error("%s", err)
-
-    if errors_seen:
-        exit(1)
-
     with create_global_lock(session=session, lock=DBLocks.MIGRATIONS):
         if show_sql_only:
             log.warning("Generating sql scripts for manual migration.")