Remove ArtGtestsTarget from TEST_MAPPING

Since If4efdfa9fe52e4ddb5c6a3ed409a27935b17cebf,
it is an obsolete name which does not exist any more.
Gtests are run via gcl config (not via TEST_MAPPING).

Bug: 184717393
Change-Id: I32fc448cf31679463c1ef13bdab304e8972630e2
2 files changed