[#649] Rename db_to_form_package_schema() -> default_show_package_schema()

This makes the name consistent with default_create_package_schema() and
default_update_package_schema().
[#649] Get rid of form_to_db_schema_options()

package_show(): Don't call an IDatasetForm plugin's
db_to_form_schema_options() or db_to_form_schema() anymore, instead call
package_show_schema(), if the plugin doesn't have that method or if it
returns None then fall back on
ckan.logic.schema.default_package_show_schema(). (Same pattern as for
package_create() and package_update()).

Delete DefaultDatasetForm's db_to_form_schema_options() and
db_to_form_schema(), add show_package_schema() that just returns None,
same as create_package_schema() and update_package_schema().