Remove mention of db_sync per backend

Many releases ago, we supported the notion of having migration
repositories per backend interface. If a backend needed to use the
database, it could manage it's own migrations independently from the
rest of keystone. That functionality was removed long ago, and this
blurb of text should have been removed with it.

Change-Id: If90e25ec556cf42322509ef28878e96120b0baad

doc/source/architecture.rst

@@ -183,15 +183,6 @@ If you implement a backend driver for one of the Keystone services, you're
 expected to subclass from these classes.
 
 
-SQL Backend
------------
-
-A SQL based backend using SQLAlchemy to store data persistently. The
-``keystone-manage`` command introspects the backends to identify SQL based backends
-when running "db_sync" to establish or upgrade schema. If the backend driver
-has a method db_sync(), it will be invoked to sync and/or migrate schema.
-
-
 Templated Backend
 -----------------