Revert "Export implementation class jars for java_boot_libs"

This reverts commit 22ff0aaf.

The workaround to support hidden API generation with prebuilts by
passing the full implementation jars through to the SDK snapshot is no
longer needed as the monolithic hidden API flag generation uses the
prebuilt flag files that are already part of the snapshot.

This change reverts that previous workaround.

Bug: 192868581
Test: - Update the prebuilts
      DIST_DIR=$PWD/dist TARGET_BUILD_VARIANT=userdebug art/build/build-art-module.sh --skip-apex
      packages/modules/ArtPrebuilt/update-art-module-prebuilts.py --local-dist=dist --skip-cls --skip-apex
      - Build hidden API flags to make sure it does not fail
Change-Id: Idb3fde6f7dcb171677316d8794a4af91ede1f7e0