tmpfiles.d: remove .Test-unix, it's obsolete

See libxtrans commit 0794b1b712a90b40e2b019c9edc6f96874493c52. The code
to generate this socket was removed 5 years ago and even before it was
conditional on #define TEST_t. There is no reference to that #define in
either the X server nor libX11's git history, or in any of the current
libX*.

Let's assume this is well and truly obsolete.