Remove api-versions-jars-dir from api_versions_base_defaults

Previously, `api-versions-jars-dir` contained `android.jar` files that
were consumed by Metalava when generating `api-versions.xml` files.
However, they are no longer used as those jars were copied into
`sdk_dir` by https://r.android.com/3454200 so `api-versions-jars-dir`
is no longer needed in `api_versions_base_defaults`.

Bug: 390113278
Test: # Before applying this change run:
      tools/metalava/scripts/gather-android-metalava-artifacts.py before
      # After applying this change run:
      tools/metalava/scripts/gather-android-metalava-artifacts.py after
      # Make sure that there are no differences.
      meld before after
Change-Id: I4f04b81bfcddfb1c8b6ffb46d72b21a9d530f4ed