fix(tests): destroy without --app flag to remove git remote

The `deis` CLI change in 0ad398f2c16f553427f393b1331f061544875c50 created a problem for the integration tests: apps_test.go expected the `git` remote to be removed, which it intentionally was not after that change. This adds a variant of the command without the `--app` flag which then acts as expected using the current `deis` CLI.

It's not clear to me how the previous change slipped by testing, but this fixes it.