Drop explicit dependency on python-subunit

It is no longer directly used by any test code in this repository since
we switched to stestr[1].

It is now installed as a dependency of stestr.

[1] aa6baae9e03d8cc8c0eed9b15a75e03a77ff3736

Change-Id: I07911d49a1c1d66c88d6eaa1e0f2e8183350449f
1 file changed