[auth] remove unit test check from PRESUBMIT.py
[auth] remove unit test check from PRESUBMIT.py

`luci-py-try-xenial-64` runs unit tests in auth_service.
https://logs.chromium.org/logs/infra/buildbucket/cr-buildbucket.appspot.com/8876000515647910656/+/u/auth_service/run_tests_python2/stdout

See auth_service/test.py for the test runner setup.

Change-Id: I8a8462a1f3035fafb310de8466dad055d57b36f6
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2277544
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py