Don't pack target tests into art-host-tests.zip.

art-host-tests.mk packs target tests into art-host-tests.zip. This
behavior has been there since the very first version
(https://r.android.com/1437898).

Today, this behavior normally does nothing because target tests are
included in the ART testing apex and therefore not visible to Make.
However, if we remove any target tests from the ART testing apex, this
behavior will start to cause problems (b/360150490), as it will pack
those removed target tests into art-host-tests.zip and continous testing
will fail to run them.

This change removes this behavior.

Bug: 357802879
Bug: 169111770
Bug: 360150490
Test: `m art-host-tests` and see that the output is bit-identical with
  and without the change.
Change-Id: I2e9abdf4ed51bd8dc91637d7502350dfafb4aa39