Revert "Change the `StandardAttribute` load method to "selectin""

This reverts commit 7f702af482.

Reason for revert: this patch was introduced to make the database
produced code compatible with PostgreSQL. During this cycle we
dropped the support for this backend. This patch is probably
introducing a race condition when retrieving the port standard
attributes because these are retrieved in a second SQL query
(lazy='selectin').

In order to keep a healthy database engine, supporting
MariaDB/MySQL, it is better to retrieve the selected field in the
``port`` retrieval query instead of issuing a delayed second one.

Closes-Bug: #2093258
Change-Id: Ida5afffb808597aa09cc62984b36189a8bf409a0

neutron_lib/db/standard_attr.py

@@ -152,11 +152,8 @@ class HasStandardAttributes:
     # pylint: disable=method-hidden
     @declarative.declared_attr
     def standard_attr(cls):
-        # NOTE(ralonsoh): the load method has been updated from "joined" to
-        # "selectin". The first provides more complex queries but it is only
-        # needed one query.
         return sa.orm.relationship(StandardAttribute,
-                                   lazy='selectin',
+                                   lazy='joined',
                                    cascade='all, delete-orphan',
                                    single_parent=True,
                                    uselist=False)