Only use schema_translate_map on SchemaItem subclasses

Only use schema_translate_map on SchemaItem subclasses

Fixed bug in new "schema translate" feature where the translated schema
name would be invoked in terms of an alias name when rendered along
with a column expression; occurred only when the source translate
name was "None".   The "schema translate" feature now only takes
effect for :class:`.SchemaItem` and :class:`.SchemaType` subclasses,
that is, objects that correspond to a DDL-creatable structure in
a database.

Change-Id: Ie8cb35aeaba2c67efec8c8c57c219e4dd346e44a
Fixes: #3924