db: Remove "support" for configurable migration backends

This doesn't actually do anything, and hasn't since we merged change
I77921366a05ba6f9841143af89c1f4059d8454c6 way back in Ocata. Drop it.

Change-Id: I623b2ffb8bdd72525241835b5d1164c275f9fe73
Implements: blueprint remove-sqlalchemy-migrate
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

doc/source/contributor/database_architecture.rst

@@ -250,12 +250,6 @@ of an *Image* object.
 Glance Database Backends
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
-Migration Backends
-------------------
-
-.. list-plugins:: glance.database.migration_backend
-   :detailed:
-
 Metadata Backends
 -----------------
 

glance/db/migration.py

@@ -19,30 +19,13 @@
 
 """Database setup and migration commands."""
 
-import threading
-
 from oslo_config import cfg
 from oslo_db import options as db_options
-from stevedore import driver
 
 
-_IMPL = None
-_LOCK = threading.Lock()
-
 db_options.set_defaults(cfg.CONF)
 
 
-def get_backend():
-    global _IMPL
-    if _IMPL is None:
-        with _LOCK:
-            if _IMPL is None:
-                _IMPL = driver.DriverManager(
-                    "glance.database.migration_backend",
-                    cfg.CONF.database.backend).driver
-    return _IMPL
-
-
 # Migration-related constants
 EXPAND_BRANCH = 'expand'
 CONTRACT_BRANCH = 'contract'

setup.cfg

@@ -56,8 +56,6 @@ oslo.config.opts =
     glance = glance.opts:list_image_import_opts
 oslo.config.opts.defaults =
     glance.api = glance.common.config:set_config_defaults
-glance.database.migration_backend =
-    sqlalchemy = oslo_db.sqlalchemy.migration
 glance.database.metadata_backend =
     sqlalchemy = glance.db.sqlalchemy.metadata