Remove NetworkStack AndroidTest.xml

The AndroidTest.xml file is not necessary as the test configuration is
generated from the Android.bp.
Because of this file, NetworkStackNextTests currently runs
NetworkStackTests.apk, so there is missing coverage.

Test: atest NetworkStackTests NetworkStackNextTests \
        --rebuild-module-info
Fixes: 147271211
Change-Id: I601d5c3eaeb6385efbf4c3cc104c447b76b515c3