backups: make listing by site only return box backups

src/routes/test/backups-test.js

@@ -37,7 +37,7 @@ describe('Backups API', function () {
         const response = await superagent.get(`${serverUrl}/api/v1/backup_sites/${site.id}/backups`)
             .query({ access_token: admin.token });
         expect(response.status).to.equal(200);
-        expect(response.body.backups.length).to.be(2); // this contains the box & mail backup, should it?
+        expect(response.body.backups.length).to.be(1); // only box backups are listed
     });