alembic: fix ef39fcd6e1cd downgrade

op.drop_constraint requires a valid field to drop the constraint on.
Without this, downgrade cannot occur.

Signed-off-by: Kevin Morris <kevr@0cost.org>

migrations/versions/ef39fcd6e1cd_add_sso_account_id_in_table_users.py

@@ -25,6 +25,6 @@ def upgrade():
 
 def downgrade():
     # ### commands auto generated by Alembic - please adjust! ###
-    op.drop_constraint(None, 'Users', type_='unique')
+    op.drop_constraint('SSOAccountID', 'Users', type_='unique')
     op.drop_column('Users', 'SSOAccountID')
     # ### end Alembic commands ###