[9.0][FIX] database_cleanup: Isolate build (#719)

* [FIX] database_cleanup: Isolate build
* Isolate `database_cleanup` into its own build in Travis file to fix #689

* [FIX] database_cleanup: Remove KeyError assertion
* Remove KeyError assertion in tests due to PR in comment being merged

database_cleanup/tests/test_database_cleanup.py

@@ -59,10 +59,6 @@ class TestDatabaseCleanup(TransactionCase):
         self.registry._pure_function_fields.pop(
             'x_database.cleanup.test.model')
         purge_models = self.env['cleanup.purge.wizard.model'].create({})
-        with self.assertRaisesRegexp(KeyError,
-                                     'x_database.cleanup.test.model'):
-            # TODO: Remove with-assert of KeyError after fix:
-            # https://github.com/odoo/odoo/pull/13978/files#r88654967
         purge_models.purge_all()
         # must be removed by the wizard
         self.assertFalse(self.env['ir.model'].search([