Remove obsolete tests

- This Android.mk is never included, because an Android.mk in a
  parent directory shadows it
- The same test is defined in
  frameworks/av/media/libmediaplayerservice/tests/, with the same
  module name, and only minor, ignorable diffs in the code

Bug: 122331945
Test: m DrmSessionManager_test
Change-Id: Ida41ec7c785aad54155bcaf9a20a7fef5ba02ffc