Alter old migrations so that they can downgrade

The Role -> User foreign key constraint was preventing roles from being
deleted once there were existing users referencing the role. I realized
it was best to just pass on the downgrade and allow the tables to be
deleted.

alembic/versions/4ea5917e7781_add_default_atat_role.py

@@ -34,6 +34,4 @@ def upgrade():
 
 
 def downgrade():
-    db = op.get_bind()
-    db.execute("DELETE FROM roles WHERE name = 'default'")
-
+    pass

alembic/versions/96a9f3537996_add_roles_and_permissions.py

@@ -169,15 +169,4 @@ def upgrade():
 
 
 def downgrade():
-    db = op.get_bind()
-    db.execute("""
-        DELETE FROM roles
-        WHERE name IN (
-            'ccpo',
-            'owner',
-            'admin',
-            'developer',
-            'billing_auditor',
-            'security_auditor'
-        );
-    """)
+    pass