cronet tests: remove most of NetHttpTestsLibPreJarJar

Since cronet_java_tests (previously cronet_aml_javatests_sources) is now
an android_library directly in external/cronet (to support "mm" in that
folder), the dependencies can be removed.

A form of android_library is still required to wrap cronet_java_tests,
so the AndroidManifest (in this folder) is forwarded properly to the
common coverage test target.

Test: TH
Bug: 278639767
Change-Id: I25c6371c64e6cfcbb48e9e3f2976d47279791f21